It depends upon whether or not your wall is structural or partition. If it's a load bearing wall, you 've got a fair amount of work ahead of you; partition, no so much. Google some videos for diagnosing which type of wall you have please.

I will! Thanks for the tip!

The red mark is the wall in question. Pictures of my attic. Can you tell which way the joists run in relation to the red mark?

I am thinking of cutting the metal stud rotten bottoms, sister with a new piece and put a new 2x4 underneath. After I clean and make everything smooth. Would that work?

With your wall running the same direction as the trusses, I don't believe it's structural. You can remove the pieces that are damaged. You can replace the components of the wall with wood or metal studs of the same dimension. You'll need a bottom plate piece to replace the damaged portion. You'll need to replace the subfloor that the bottom plate rest on if it was damaged. And you need to examine what's under the subfloor. Could be joists that may need repair.

Great! Thank you. Very relieved. I'll check the subfloor. They put a piece of wood inside the bottom metal plate. Do I need to do that? Or just replace the metal plate and sister with new metal studs the bottom of the studs that are rotten?

I'd rebuild including the lower wooden filler.