Q.Treating severe root rot old man cactus
I have an old man cactus which is 26 years old. Over the past few months, I noticed that the cactus was growing at a lean. I thought that it wasn’t getting enough sun and was growing lopsided towards the sun. I moved it to a different position to get more sunlight. It seems now that I was wrong and my cactus was getting far too much rain water and has rotted from the base. The cactus is half its thickness and looking quite poorly. The base is black and grey and appears to be rotten.
How can I save my cactus? Will I be able to cut of the rotting base and let the cactus dry out for a few days before re-potting?

You could take cuttings from the cactus and save the plant.
Here are a few links for you.
https://www.gardeningknowhow.com/ornamental/cacti-succulents/old-man-cactus/old-man-cactus-care.htm
https://www.gardeningknowhow.com/ornamental/cacti-succulents/scgen/propagating-cacti-and-succulents.htm
https://www.gardeningknowhow.com/ornamental/cacti-succulents/scgen/removing-cactus-pups.htm