If you want your ring to fit tighter, first, make sure the ring is clean and dry. Then, you can use a small rubber band or a piece of thread to wrap around the back of the ring and loop it over the front. This is a great alternative and will help as a ring size adjuster.
As a temporary solution, you can try bending the ring a bit to make it fit properly. If all else fails, the only option is to take it to jeweler soldiers to have it professionally resized as a permanent solution.