I changed the battery in my esq swiss watch and can't get the back on the watch. What is the trick to re-installing the back

Ooh, ESQ by Movado. These case backs are often pressure fit, just like Movado, and are notoriously difficult to reinstall. It sounds like it is going to take a watch press to install your caseback. This is an expensive tool which any jeweler that changes batteries will have. There is no real safe way to this without a press.
Now, I have just supposed that this is a pop off; if it is a screw on back then it would have had to have been unscrewed to get it off. If it is a screw-back then you will likely need to get a caseback wrench to tighten it down enough.