Most DIY/harware places will sell varnish stripper solutions. Combination paint/varnish stippers are also common. Alternative a heat gun will strip the varnish but there may be a fume issue depending on the age of the varnish. Also it is quite easy to burn/mark the wood with a heat gun.
This will get rid of the varnish but may not get rid of the stain. Most stains seep into the wood to some degree and are very difficult to remove.
If you want the wood to be darker than it is currently is then no problem simply restain with the new shade. If you want the wood to be lighter you will need to sand the wood once the varnish has been removed.
I suggest you try a small area to see if you can get the desired effect