I quickly googled how to change a SRAM rear mech set-up. I bought my bike 2nd hand and it's the first time in 20 years I've moved away from Shimano.
I watched a youtube video on how to thread the cable through the shifter. It looks a little more complicated than rapid fires, but by no means 'hard'. However, all of the comments below were about how the lever will explode in my face and I'll need to by new.
1) take the shifter off of the bar clamp (with the shifter in the lowest gear)
2) thread the new cable through in the reverse way the old cable came out
3) set up the gears as I would have a Shimano set up, but with the only barrel adjuster on the shifters, no adjustment on the mech itself
p.s. I have new cable outers too (cut to length). I assume, that I simply pop it into place.