My Apple Watch won’t update
Why won’t my watch update? What do I need to do?
Apple Watch Series 3, watchOS 7
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FWIW, a lot of people have problems updating a series 3 watch due to insufficient memory. in some cases, unpairing the watch, completely erasing it, and then updating and restoring the backup will allow you to update it.
Unpair and erase your Apple Watch - Apple Support
Another option (maybe even better, if possible) is to take to make an appointment at the Genius Bar in an Apple Store near you and let them update it.
Genius Bar Reservation and Apple Support Options - Apple