Carefully bend th teeth in the setting away from the stone. Use needle nose pliers or jeweler's tool made for removing stones. Diamonds do not scratch, so you don;t have to be overly careful with the stone.
If it's in a prong setting you can pull back the prongs and remove the diamond. If you are not feeling confident about doing this or if you don't want to scratch the ring that the diamond is in you can take it to a jewelers and they will do it for you. Most don't charge much.
