Locate the leak and speak to a professional about patching it. Your paint will continue to have problems until you actually repair the leak.
Set a tarp under the work area to expedite cleanup.
Cut around the sagging paint with a craft knife. Press only hard enough to remove the paint, not to damage the sheet rock beneath. Peel the saggy paint away.
Allow the wall to dry for a full 24 hours before you do anything. Applying paint over soggy drywall will lead to negative results.
Sand the edges of the remaining paint lightly to smooth them out. This will make it easier to blend the new paint in with the rest.
Paint over the wall with white primer first to preserve the pure color of the actual paint. Allow it to dry before you continue.
Paint over the dried primer with a shade of paint that matches your walls. Allow it to dry and apply another coat to make it more solid. Do not touch the wall until it dries thoroughly.
Roll up the tarp and empty it of paint chips as well as splatter over the trash bin.