@australis When I see a cactus yellowing like that and it is soft when squeezed I think it is dying. chop it off all the way down till you encounter green, healthy tissue. If no healthy tissue then chop it off at ground level.
The second cactus in the pot looks green & healthy. However, it is sprouting from where the original cactus was topped off. I would cut off the healthy top section right where it is sprouting from the bottom section. Right where it is the narrowest. Let the cut section heal or callus in a shady, dry location like your garage for a week or two. I have read somewhere to let it heal one week for each inch in diameter of the cut end.
Then cut a couple inches off the top of the old base section and leave it alone. Eventually it will sprout a new growth from the old base. I cut off the top to encourage growth from a 'new eye'. These plants always seem to grow out of the top where it is cut off. Therefore you need a new top (exposed eye) for it to sprout a new growth. You may want to hide this old plant out of sight since it doesn't look 'pretty' any more.
Once the healed top section is ready, it is time to plant it in a new pot with fresh mix. Use your normal, well drained cactus mix and fill up a gallon size pot about 1/2 way. stick your cactus into the mix an inch or so and support it with stakes. I use three tall stakes and tie them together at the top
Wait a week or so before watering and then water whenever it is dry. Do not over-water, but you do not want it to become bone dry since these are baby roots. When you see roots growing out of the bottom of the pot it is time to pot it up into its new pot. This is a slow process but worth the effort.
If you leave the cactus as it is now (growing from the cut off base) it will become top heavy and break off by itself. Not a pretty site since they can dent and break into pieces when they fall.
