removing old panneling found rotted 2x4s next tofireplace and a window,haven't tried anything yet,since this is an exterior wall too,do I have to build a temp support wall,or will the fireplace support thewall while I pull out the old and put in the new.the outside of the house is brick

A support wall would be necessary if the ceiling beams are seating on the existing wall.

If possible put in new 2x4's before removing old rotten ones.

Would also find out why studs rotted and correct the problem. There must be water getting in to cause the rot or vermin.

Chuck