With that 6-key case back, it would normally be relatively easy to open using an adjustable watch case opener.
If not, then the rubber gasket has most likely perished and locked it up.
The only other way I know to open the case is to super glue a large nut to the case back. You then place the watch in a watch case holder with vice and turn the nut using a spanner. Once you've removed the case back, you soak it in acetone until the super glue has dissolved and the nut can be removed. I tend to leave this to my watchmaker.
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