A case can guard your iPhone against many dangers, from superfluous nicks and scratches to severe water damage and falls. Cases also provide a medium for customization, allowing you to give the device a splash of personality. If your case no longer fits correctly, you will need to tighten it, as an improperly fitted case will not be effective. The best method for tightening your case will depend on the type of case you have, and some cases may simply need to be replaced.
Adjust any straps that your case may have. If a strap has come loose or stretched, sew it back in place, adjusting it to fit correctly again.
Insert a soft material, such as tissue or paper towels, into the place where the case has come loose. If your case is loose at the top, inserting a stuffer into the back will help tighten it in all places. This is best for hard cases, though it works for silicon and fabric cases as well.
Re-sew the seams of the case if it is made of a sewable material. You may need to completely remove the seams and sew them slightly further in to make the case appropriately tight.
Wash the case in cold water to shrink it if it is made of a stretchy fabric.
Boil a silicone case for 30 seconds to soften it, then dunk it in cold water. Quickly dry it and fit it to your iPhone. It will reshape like a mouthguard.
Avoid getting your iPhone wet or overly hot, and avoid using materials that could scratch the phone. If your iPhone's own shell is loose, take it to your local Apple Store, as this is a manufacturing defect.