How do you fix too much salt in mashed potatoes?

You could add milk, or heavy cream, or butter, or sour cream.

OR

Mix up some plain instant potatoes according to directions, except omit the salt. Add to the mashed potatoes and blend well.
A number of ways - Add more raw potatoes having cooked these without salt and MASH in for serving.

Use low/no salt butter to cream the potatoe, add some milk to reduce the salt balance.