Since you are asking this question, I would get a carpenter or contractor to do this for you....

But here is some info that you need to consider either way.

First - Since you are increasing the rough opening (larger header), did you check with an engineer/architect/county inspector that your headers are large enough for load above each window?

Secondly - most outer walls are load bearing, and you should support the floor joists for the second floor in front of the each header you are switching out. Also load carried by the temporary header, supports, and their associated jacks need to be resting on something that can take the transferred load. Like a slab on grade foundation, floor joists below, etc. Temporary supports in a basement maybe needed below the temporary support on the first floor to transfer the load from all the way to the foundation just like the load-bearing wall does.

Thirdly - Did you get your building permit?

If you still have any questions at all about how to do this safely without having your upstairs cast iron tub fall through the floor Get someone else to do this for you.

BTW - I will be doing something similar myself this morning, but I am only increasing the header by 4". I used to rough frame houses when going through school, so I have enough knowledge to shoot myself in the foot. I still needed to check with an engineer/inspector to make sure my headers would be larger enough for the load.

Depending on what your walls are made of will determine the bracing needed for your header application. In most cases we never use any supports unless the opening exceeds 8" or the home is built with styrofoam exterior sheathing. We also take into consideration what sits above the ceiling on the next floor, like a cast iron bathtub. In these cases we will support the ceiling with two pole jacks and a 6x6. I don't recommend taking a lunch break as soon as you get everything cut out. We always pull out the old and throw up the new stuff immediately. This gives the floor above you little time to sag or drop. We do all our prep work ahead of time so all our wood is cut and our headers are done and ready to go. This makes the assembly of the new opening much faster.

I agree with Endyii on checking your local codes and getting a permit. Almost every city requires a permit for altering an opening. The size of your header is determined by the width of your home and how many levels it has. If you live in an area like I have here in MN, you have to take "Snow Load" into consideration on the header size. So make sure you calculate all these together and get the proper sizes. If this seems a bit more than you wanted to do then I would suggest hiring a contractor to make these changes for you. Good Luck!!!