Is it possible to get some help?

Thx for ur reply :)

It's been first winter, since I have that plant. Is it really natural that it got so shriveled? Or is it stem rot? If soil is too heavy, shall I w8t till summer to replant it or do it now in winter time?

Hmm,bad news. Shriveling trunk on any plant is near a sure sign of death. Fungus,rot,too much or not enough water..once it gets to that look..

If I was you? I would take it out and remove that heavy peaty potting soil and fill in around with 70% perlite and some potting soils. Give the roots air.

I cannot tell from the image if there is discoloration along with the shriveling. To me that would be a likely indicator of rot. Shriveled branches by itself are only a clear indication of a problem. What happens next is the deciding factor if rot sets in.

I would treat it like you were attempting to root a fresh cutting and see if it can be saved. cut off all but one or two of the youngest leaves. Dry and warm might make a difference. Use a heat mat if you have one...if not go buy one. Stano's advice to get it out of heavy soil and into a perlite (or similar) mix is seconded.

I think you will find a rotted mass of brown roots. The hope will be the rot has not gone past that point. It may be salvageable.

Guys...in the end plumeria was all fine... I cut one of stick to find out very nice white flush! Omg. I hope with my cut I did not kill her 0.o

I was going to post a question like this too when I found yours.

My Plumeria looks just like yours and it is its first winter too, and
it started to become slightly soft in the middle of the stem. I hope it
will keep doing fine.

Here is a post I wrote about a month ago about my Plumeria :)

About
the cutting, I don't think it will cause any harm to the plant, I've
read once about rooting Plumeria cuttings, so you can keep it and plant
it in spring and have a new tree, search for rooting Plumeria cuttings
:)

After posting this ? I was given a cutting...that had shriveled some. Still the darn thing sent out roots. Now Its planted. So,I learn everyday too.

Yess I kept cut and put it str8t to cactus/normal soil (proportion 3/4). Hope it will do fine, but now its winter here and not so much sun neither heat :( guess not the best time to start "new life"