Replace internally routed and housing
There are many different shift cable and housing arrangements found on internally routed frames. For example, one end of the spectrum is bare cables running all the way through the frame with plastic cable guides to route them around the BB, and the opposite end is just a continuous run of housing routed internally. The best procedure is dictated by what kind of configuration you've got, or in some cases what options the frame offers. 

Replacing just the cable is easy on some frames but enough of a pain on others that it's better to replace the housing too so that you know everything is fresh and perfect and you won't have to do it twice.

Your best friend when doing this is often a supply of cable liner, available from bike shops, which you'll cut to length, use some tricks to feed through the frame in place of the old cable as you pull it out, and feed the new cable into.