It is best if the current clear coat is removed. With this finish removed the process becomes much easier. You could experiment with some paint and varnish strippers and see how they work. You don't have to remove the stain, just the clear coat.
With the clear coat removed, a gel stain can be used to recolor the wood. This method is the easiest to do and doesn't require any new skills. The stain will be easy to work with.
If stripping of the finish isn't possible, you could try a gel stain over the current finish. This is more difficult to do as the gel stain will be floating on top of the finish. The preparation for using a gel stain over a clear coat is quite basic.
1) Wipe down all surfaces with a deglosser. I like Paseo, but there are others on the market. This will remove any oils and degloss at the same time.
2) Lightly sand with 180 grit sandpaper or sanding sponges. This will help with adhesion and control of the gel stain.
3) After sanding, wipe everything down one more time to remove any dust.
Now you can apply the stain with a soft white China bristle brush.