Battery replacement
Should I erase my Apple Watch before leaving it at a repair shop for a battery replacement
Apple Watch, watchOS 4
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Yes... and also remove the watch from your Apple ID so Activation Lock is cleared. This will let them load software and test it if necessary. The other reason to erase is that unpairing automatically makes a backup of the watch to your iPhone. If anything goes wrong during the battery replacement you have a current backup available to be loaded, even if they lose it in the workshop and you end up with a completely different replacement watch.