You can do it with a three pronged case wrench. I haven't done it in many years, but it can be done. If you have any of the triangular prongs for the wrench, use those and tighten them in until they are as snug into the grooves as you can and it should just rotate off.
If you are not comfortable trying it, a jeweller, especially one that handles Victorinox watches should be able to handle it for you.
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