Let handy lady tell you how to fix it because i fixed mine myself.
1: Make sure it is not termite damage. If it is, treat the termite first.
2: No need to take the whole part off the door frame. Remove the rotten wood, then go to homedepot buy a compound. Can't remember the name of the powder which is in a can, costs about less than $5.
3: Mix it with water according to the instruction. Make it into a ball shape, then fill in the hole. Smooth the area out and wait it to dry.
4: Sand the surface and then pain it. It will looks like brandnew.