Did mine a while ago Geo, it wasn't hard.
IIRC the top cover screws are phillips head, you want quite a small phillips s/driver.
To extract the gear cable, undo it at the derailleur end, then (again IIRC) push the cable into the shifters a bit, you'll see the nipple on the end of the cable, inside the shifter. 'Tickle' it out of its location with a small pointy thing then pull the entire cable out. It may just come out anyway, mine was a bit tight.
Reverse procedure to install new inner cable, just make sure the nipples located properly.