Follow this guide to reinstall the back cover and replace the adhesive.

Be sure to turn on your phone and test your repair before installing new adhesive and resealing the phone.

If desired, you may reinstall the back cover without replacing the adhesive. Remove any large chunks of adhesive that might prevent the back cover from sitting down flush. After installation, heat the back cover and apply pressure to secure it. It won't be waterproof, but the leftover adhesive is usually more than strong enough to hold.