To make a ring smaller, take the ring to a professional jeweler and ask for it to be resized. To resize a ring, a jeweler cuts metal from the base of the ring's band, pulls the edges together and solders them shut.
There are certain elements that make resizing a ring harder and more expensive. Rings that have jewels, particularly jewels that are heat-sensitive, can make the resizing process challenging. Ask the jeweler to redo the work if the jewels are loose after a resizing. Other elements that make resizing challenging include engravings and thin bands.
Making a ring larger is much more challenging than making one smaller. In order to make a ring larger, a jeweler can stretch the metal. However, this may weaken the metal as it becomes thinner. Alternatively, the jeweler can cut the band, pull it apart and add a metal bridge to the empty area.
To make a ring smaller at home, use a silicone adhesive such as Loctite StickN'Seal Indoor Adhesive. Place the ring on a piece of newspaper, cover the inside of the ring with silicone and let it dry. Try on the ring. If it does not fit, apply another layer of silicone. Repeat this process until the built-up layers of silicone have made the ring smaller.