I believe this cactus is a cereus jamacaru but I am not 100% sure. I had it in relatively low light room but across from a south facing window in the sunniest spot of the room. I watered it once before I noticed something was wrong. It started out by turning yellow and a little brown at the top and now it is starting to turn white. I repotted it and used a soil that has better drainage and the pot it is in has a hole in the bottom too. I even transferred it to different levels of light (maybe too quickly from what I've read) I also believe it has scale insects. After researching how to remove them, I started the process this morning and I have also ordered a cacti insecticide just for future prevention. Are there any other diagnoses? I don't know if I'm correct and I am wondering if there is a way I can cut the bad parts off or if it will even recover. Any Ideas? The attached picture was last week- it looks worse now and has since been moved to a less sunny place- it was out in the sun for a day or two.
First, chop off anything that's not green. If you're lucky what's left will send out new offsets. If you chop the not-green part off, and you see yellow/brown in/around the central bundle it's likely to be a goner. Keep chopping until you don't see rot.
I get the sense that you haven't had this plant long. If that's true, whatever happened happened well before you owned the plant.
Most cactus don't do anything fast, and that includes dying.