Only way to replace/remove the evaporator is to remove the complete dash assembly in order to get to the heater assembly. This involves a weekends work and the help of a couple of mates as the dash is cumbersome and heavy. Full removal instructions on RAVE. There is no easy way to do it which will take 20 minutes despite the people who will say "I did it easily whilst having a cup of coffee".
What I will say is, whilst you have got the dash out, change the Blend Motors if you have the Book symbol on the HEVAC and as a matter of course, replace the heater "O" rings. These parts are easily accessible whilst the dash is removed and you have a golden opportunity to do them rather than lay on your back struggling with a saw, made up tools etc. to get to them.
Are you sure that the evaporator is shot and not the condenser. The heater matrix on the P38 is pretty bullet proof, as is the matrix....common problem is the condenser. Some people on here have had success with "Plastic Steel" repairs, but I have preferred to just fit a new condenser. and I have fitted two in four years.