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Use a jewelry vice (with padding) and a metal file and do it by hand. Don’t use a Drexel, or you will likely remove too much. Take your time, check often. I’ve done this a few times with 20mm end links to get them to fit a 19mm lug size. Only time I ruined one was using a Drexel. Took off too much too quickly.
better to do it slow than fast, unless you know what you are doing.
I’d pay someone to do it.
By hand? …….or are you talking about the watch