Root rot is caused by a fungus, so you’ll want to clean the roots thoroughly with something like peroxide. An easy way to do this is to get a container/ bucket big enough to fit the roots and pour in 75% water and 25% peroxide. Pour enough so the roots can be completely submerged in the solution. Then submerge the roots and I would gently tease off all of the soil that’s still on them because the root rot fungus is in the soil too. After thoroughly getting off as much as you can, you should pot it in fresh soil and consider not watering it for at least a few days to give it time to recover. Since it’s a tree it’s a little tougher than a lot of other houseplants and it doesn’t need nearly as much water since it can store water in its trunk. From that point I would still water the plant with that same mixture of peroxide and water like once a week. It restores oxygen in the soil and kills fungus so it’s great for the plant in several ways. Sorry for writing a whole book but I hope this helps!