If the fascia is rotted, you could repair the wood with polyester filler in the form of a wood hardener and filler. You would just need to remove all of the rotted wood with a sharp tool. After removal, coat the area with wood hardener and finish off with wood filler and press into area with a putty knife. However, this fix is only a temporary fix for fascias that are mildly rotted.
In most cases, the fascia needs to be replaced. An overview of this project is quite simple — all you need to do is remove the damaged fascia board, install a new board, seal the seams, and finish with a coat of paint.