One of the handles is loose. I can't find any screws to tighten. The only thing that can be unscrewed is a top of the lever, but I can't see any screw inside the hole. This is an "elbow" type handle. Please help.

Look for a Allen Head Set Screw in the hole. Good luck, Tom

Some faucets, Moen comes to mind, do a great job of hiding their screws under what appears to be just a logo sticker. If you see such a logo try prying it out with a sharp pointed instrument like an awl or pocket knife tip.