[lolife99]

320 would be fine.
I use an electric palm sander,... but an air sander would work too.
You can finish off hand sanding to get out aall the "swirls" that the electric sander would leave.
You don't have to get off all the stain,... just the clear.
So it should come off fairly easily.
Apply your new stain with a foam brush or a rag.
Wipe off the excess with a clean dry rag and let it dry.

I usually spray the clear because I have a buddy that has all of this stuff in his cabinet shop.
But you can brush it on just as easily.
Apply as many coats as it recommends on the can.
You may have to sand the clear between coats to knock off and bumps or nubs in the clear.
If you have the patience to do bed wood,... this will be easy for you.