Sanding particle/pressed wood
You can sand it to smoothness, but you will remove the finish on it. If you do, I would locate a lumber supply that handles wood veneer. Cut to size 3/4" over on all sides, contact cement both surfaces, let dry to the touch and stick it together remembering that you only get one chance to stick it. Get a hand laminate trimmer, trim the overage, sand lightly to finish and use a marine polyurathane called Helmsman to seal. This stuff is the best. You'll have no more problems with water damage.
Good Luck, Len
If the warped spots are not too bad (too high), then you might be able to sand them down and re-finish the top. However, usually the veneer is as thin as it can be already, such that sanding it any further will make it unstable and subject to cracking/shredding.