A valuable ring should always be resized by a professional jeweler. A simple ring with no stones can be made approximately one size larger by hammering it down on a mandrel to stretch the ring. If it needs to be made much larger than one full ring size, it will need to be cut open and a matching metal needs to be welded into the ring to expand it. It will then be polished so that the cut will be virtually invisible.
To make a ring smaller, a small piece of metal needs to be removed from the ring, and then it will be welded back together and polished.
A ring with stones must be carefully sized to avoid crushing the stones if the ring is made smaller. If the ring is made bigger, the settings that hold the stones will be expanded so the stones may fall out.
A ring guard may be used to make a ring smaller, but this is not a long-term solution for a cherished or valuable ring. A plastic ring guard can fall out, or may require glue to hold in place. A metal ring guard is made from flexible metal and will bend or fall apart with constant wear of the ring. The ring guard can also been seen from the outside of the ring, so it is not very attractive.
Metal ring guard