All you need is a couple rasps (flat and rounded) and some sand paper. Shape it the way you like, sand out all the tool marks and make it nice and smooth, then apply several coats of finish. It might be a little tricky to get the finish to match the rest of the bow unless you refinish the whole thing. I use minwax wipe-on polyurathane on the bows I build.
One note of caution...If you reshape the handle yourself, you will likely void any warranty from the bowyer.