In changing shift cables on STI brifters, I release some brake cable tension so I can pull the lever all the way to the bar, put a rubber band on the lever to hold it open, and then take the cover off (some have it on only one side, some on both) revealing the cable hole. Of course, I would have shifted to the lowest cog/ring to release cable tension and line up the shift mech in the brifter to 'zero' position. Then it is an easy task to release the cable pinch bolt at the derailleur, remove the old cable, insert the new cable, pull slack out of the cable, tighten the pinch bolt, and check shifting, before replacing the cover, and releasing the lever. Finally I readjust the brake cable tension. Now a test ride is in order!