You take off the old finish with either a stripper or by hand. Then you gently sand the block until it is even and all of the old finish is gone. Then you can apply a stain to it, which ever color you want. Next put a lacquer on to protect it. Make sure it is food approved.
Wash with soapy water and a plastic scrubby to remove all residue. Rinse and dry. If needed, lightly sand. Seal with food-safe oil used for wooden cutting boards and bowls.
You will first need to sand the surface to make it smooth and remove any stains. Start with a coarser grain sand paper and then work with a smaller grit to smooth the surface. After this, apply a food grade mineral oil to coat the board.
This is very hard work, because blocks are made with endgrain upwards. I would take it to a local woodworker for bench planing. If the block has a dip in it, you could start the rough work by taking the edges and corners down with a handsaw or rasp.