If you need to resize a ring that's too big because you've lost weight or because you've got a new ring that's not your size, there are several options. Some are temporary solutions, while others will permanently change the ring's size.
Use a fabric Band-Aid (not plastic) for a cheap temporary solution. Take the adhesive off one half of the bandage and place the sticky part on the inside bottom of the ring band. Use cuticle scissors or a similar tool to trim off the excess fabric. This modification should last for several weeks, even if the bandage gets wet.
Purchase a plastic ring guard for a more durable and long-lasting adjustment and slip the ring guard inside the ring. The flexibility of the plastic will allow the ring to slide easily over your knuckle, and then fit snugly at the base of your finger.
Have the ring professionally resized by a jeweller for a permanent refitting. The jeweller will typically use one of two approaches: the mandrel method, in which the ring is compressed with a mallet; or, if the ring needs to be substantially smaller, cutting and resoldering the ring carefully so that the work is seamless.
Having your ring professionally resized is the best option if the ring is valuable and you plan to wear it regularly. If you take the ring back to the store from which you purchased it, the jeweller may do this for free.
It can be difficult for jewellers to resize rings with jewels or ornamentation on the sides of the band. Check with a professional jeweller about the best options for your ring. Ring damage is possible, so choose your method carefully.