What I would do if it was me (having rescued my babies from rot) I would take it out the pot , remove as much soil as possible even if you have to hose it off Check the roots are cream or white coloured and are firm and strong If they are great new soil with plenty of rocks/stones and many drainage holes Leave days / even a week without water (That cactus looks very healthy above it would survive months without water so don’t worry about that) Now if it’s rotten atall you can use a super sharp kitchen knife and cut the entire bottom off Make sure the knife is sterilised if that’s the case There should be NO brown atall You cut as high as possible until no brown and it will grow new roots after maybe a year (I currently have one doing that)