You will have to use a paint/varnish stripper to remove the clear coat. After the stripping some sanding will be necessary. The amount depends on how well the stripper worked. It is important to remove the varnish. The new stain has to penetrate into the wood in order to cover the existing stain color.
It is easier to go with a darker color than a lighter one. Use a gel stain for the new color. It will easily cover the old stain color. I like Old Masters gel stains. The have a neutral base that can be mixed with their gel stains to lighten the color.
Try the paint stripper, sanding and staining on the back of a door. It is best to practice on something that is less visible.