to RESTAIN, you'll need to sand until you reach the 'unstained' ( natural color ) part of the wood; Labor intensive; have you considered using a gel glaze or even painting ?
Stains are just that .. they are absorbed into the wood grain itself, not just sitting on the top like a Glaze or Paint.
Take a door front or drawer to the home improvement store paint department and ask for their advice on paints, stains, trim and more.
You will need to sand them down to remove all the old varnish and stain first or the new stain will not absorb into the wood and your doors will look patchy.
Are you wanting to change the stain color? If so, then the above comments are how it's done. If you have worn areas and want to restain to even these out, check out Howard's Restore a Finish. I used that on my old bathroom cabinets, it was easy and worked great.