How do you fix a brown cactus?
Techniques for troubleshooting cactus problems
Then, cut off any yellow or brown tissue with a sterile knife. Allow the cactus to form a callus over a few days, then place it in fresh cactus soil in a bright, warm spot, watering sparingly when the soil dries out.
Prepare a pot filled with soil for cacti and a stake. Put the plant here on, held upright with the stake, without burying the base for more than 1 cm. From time to time moisture the soil until the plants starts to grow again. Then you can water as usually.
What does an overwatered cactus look like?
Black or brown spots and mushy stems are telltale signs of an overwatered cactus due to a fungal infection in the soil. If you notice these physical symptoms, it’s time to act.