I think you are asking about re-varnishing. If so, Sand the old varnish lightly, clean off well with a clean cloth and then varnish again. When re-varnishing, it is best to do at least two coats for a better finish. Do not shake your varnish as you will get a lot of bubbles you do not want. Just stir it with a paint stick continually, as you work.
Like Joy says. Use a damp cloth to wipe down the wood.
use a quality wood stripper and strip all of the varnish off. It will more than likely take multiple applications to remove all of the old material. You can use a medium bristle brush to get the finish off from areas with decorations cut into the wood. After removing the varnish, wipe down with mineral spirits to remove any excess and allow it to dry. sand any areas that you want to remove damage from and re-varnish.