Black is never good. It looks like there is rot. It spreads usually, until stopped. I do not know of any other way but cutting slices off until there is no black left at all.

Can't tell for sure, but the soil looks wet in your photo. Jade is a succulent, and they do not need constant moisture - that is what causes the rot.

Jades also need good light. One side looks good, so there is probably plenty of it coming thru the window. It is best to rotate the planter, so plant gets light all over. How long has it been in the same spot? What is the exposure of the window? How long has it been in that same container and what kind of soil is it potted in?

I got the plant 2 years ago, so I am not sure what soil it has; however, I just put some succulent mix on top today just before taking the picture, which might be why it looks wet. My husband and I wanted to re-pot it, but we really have no idea where to get a new pot that’s bigger than this one. It has been in the same spot for the two years that we have had it. Both sides were lush and big but last winter, I lost most of the right side. I can’t seem to figure out the watering ratio in the winter it seems. The window faces east, so it gets morning sun.

That loss could have been caused by rot. From what I see in the photo, it is quite bad - JMO.

You don't need bigger container, even smaller would do. Friends of mine have enormous jade that is at least 50+yrs old, and it has been in same pot for at least 15yrs if not longer. It is too big to be moved anymore...

Are there any drainage holes in the container? I am only guessing, but really think drainage is bad. Both container and the soil. Succulents need to be potted in well draining mix. Again, just guessing, but doubt that soil is. I would have added lots of sifted perlite or pumice rather than soil. Many ppl use 50/50 mix of sifted perlite (or pumice if available) and Cactus & Succulent soil. I have jades - not as old and big as yours, and I grow them in a mix of about equal amount of sifted perlite and grit, with very little soil added, maybe 10-15%.

I would get rid of all that rot asap, and save the plant. It looks old (that is good!), with thick and gnarly trunk - all very desirable features. I believe it would be quite pricey to buy plant like that - obviously depending on where you live. If in area that they can be grown in ground year round, it would be worth less I believe. In cold zones (like mine) it would be quite expensive.

I am surprised that it looks as lush as it does (at least on 'healthy' side) getting only morning sun...but again, I do not know your general location.

There are drainage holes in the bottom. I live in Alberta, so it is cold. I did add perlite and sand to the soil in the top layer. How do I get rid of the rot? I cut off the branch until I didn’t see anymore black.

Cutting rotting parts off is good. Could you pls. post a photo of that branch after cutting? How about spot on trunk where it broke off? Is there any rot? Could you post a photo of it pls? BTW, is the broken off branch same as on right in the container?

I never use sand either. It is usually very fine, and interferes with drainage. (For me to use sand, particles would have to be no smaller than 2mm - so it would be really fine grit.) That is why grit and sifted perlite (or pumice) is much better. I use chicken grit, 'grower' size, here is pic of it (brand doesn't matter):

But even C&S soil + sifted perlite will work; mix should look like this:

I am glad to hear there are drainage holes. That container looks good, and should be plenty big for number of years. But it is quite large, so it hold lots of soil. If all that soil retains water, plant will most likely rot.

It could be all fixed, as soon as the rot is gone.

I am not sure if I should cover it with dirt or let it dry out.

Is that where it broke off? Do not cover with anything. I would actually pull some soil away, all around. Maybe 2-5mm or bit more. It will callus, and most likely you'll get some new growth from the stump later. Is there more than just one? - I can't tell for sure.

How about pic of the branch you said you cut all of rot off?

The branch fell off a few weeks ago, so I don’t have it anymore. Yah that is another small stump. I am not sure if it is connected to the one branch that is still left on the one side.

I am not sure I understand exactly what is happening, but this is what I would do:

I would try to repot, using new mix, or at least get as much soil out of the pot as possible.

I wouldn't use any sand, but get a bag of perlite. This should be sifted to get rid of smallest particles and dust. I would mix lots of perlite with C&S soil, to get mixture similar to what the photo shows.

I would not cover stumps with the mix. If separate from the other parts of plant, I would lift the stump(s) higher, so they are above the mix line. (If you can repot, get all old soil off the roots. You may be able to reposition plant if needed.)

I would make sure there is no rot left - any spots, lines etc. that are dark need to be cut off.

I would lightly dust cut surfaces with a fungicide, either flowers of sulfur or powdered cinnamon (natural fungicide), unless it is already well callused.

I would fill the container with new mix.

Before any watering, check if the mix has dried up: use a bamboo/wooden skewer or chopstick. Insert deep into the mix , at least 3/4 deep. Leave it in the mix for several minutes (5-10 would work), then pull out. If stick is dry, water. If it is damp (feels cold) and may have some soil particles sticking to it, do not water - there is plenty of moisture. I am only guessing, but it may take 10 days - 2 weeks between watering. Possibly longer. Lower the temps, less often plant needs watering. And winter is their dormant period anyway, so growth usually slows down.

Plant will most likely grow new shoots/branches from the stumps. It may take a while, but if no rot left, I am confident it will. I would turn the side with less leaves toward the window.

Thanks for your help!

It looks beautiful!..newbie here and I'm quite impressed with how the rescue squad[lovingly,of course], has just stepped right up,and walked you through that rescue.kudos to all of you!,I feel connected! Great job..