I found a "battery" website that had some suggestions that should be typical of any snap back case watch.http://www.online-batteries.co.uk/whichb...
specifically they said that when you try to press the back on it must be flat (not one edge down as I had been trying) Put the watch face down on a non scratch surface and press with both thumbs evenly. If that doesn't work a press will be needed. If you have a vice a couple of blocks of wood to distribute the load on the front and back may work, otherwise take it to a jeweler with the proper equipment.
The thumb routine didn't work for me. I have a vice but figured many wouldn't so I tried something else first. I put the watch face down on a piece of wood. I found a small 2" long scrap of wood (3/4" x 1.5" approx) that spanned the back. I centered the back (aligned properly) placed the block of wood squarely and centered across the back and tapped the end of the block with a hammer. First try didn't work, second one did. I used a rubber mallet as the hammer but a regular hammer or other heavy object would work. The trick, I think, is to get everything lined up and square so the blow is centered and even.
Also, I'd suggest cleaning the back and the groove in the case and be sure the rubber gasket is properly positioned. This will give you the best chance of retaining the waterproofing. And remember to check that the watch is running before you do this. Some need to be reset (usually with a very small screwdriver or pin) after replacing the battery. There should be a small hole with some printing next to it. You may need a magnifying glass to see/read it.
Hope this may help you;
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