There are two ways to do the handrails. First you can remove all of the finish either by sanding or chemical remover. Them you can stain the handrails with a darker stain. The second choice would be to use Minwax Polyshades, which is a stain and urethane mixture and this would be the easier way to go and will give you good results.
The spindles will need to be sanded (wipe sanding dust off with a rag dampened with paint thinner)then prime with an oil based enamel undercoater and finish with a acrylic semi-gloss trim paint.