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How to change or remove the band on your Apple Watch
It should only take a few seconds.
It should slide pretty easily, but occasionally the band does not release immediately. If this happens, Apple suggests pressing down on the lock button again and ensure that you're holding it down firmly. Pro tip: Whenever I remove my band from my Apple Watch, I like to clean my watch with a little bit of rubbing alcohol or mild soap and a cotton swab.
Or read this on Mashable