Losing one’s Apple Watch settings and data can be a pain, and I’ve found with replacement devices it’s sometimes not obvious how to restore your Watch, as the previous Watch’s backups don’t appear.
If you break or damage your Watch, Apple are pretty cool with things if you’ve got AppleCare+. Back in February, my Ceramic Series 3 developed a fault, and with two days they’d sent me another one.
The process to switch the Watches around after that is pretty simple:
Remove the older Watch from the "Find My iPhone" service. You can do that using the "Find iPhone" app on your iPhone (by selecting the Watch and choosing "Erase Watch", or by using the "Find My iPhone" site).
Unpair the older Watch. There's no need to explicitly create a backup, as your iPhone automatically saves a backup of your old Apple Watch when you unpair it.
The older Watch will reset and erase itself. Once that's done, power it off.
Before your do this, it makes sense to ensure your Watch is running the latest version of WatchOS.
At this point, the old Watch is completely removed, and the new Watch can readily be paired. Hopefully you put it on to charge at the start of this process, although they usually arrive fully charge and they hold charge very well whilst in shipping. Mine arrived with more than 70% left, which was great.
Turn on the new Watch. It'll eventually present a setup screen, and you can follow the usual Apple Watch pairing process.
Once you get to step 4, you'll be expecting to see the backup you've just created of your old Watch. But what if it's not there?
Usually the answer is simple, although not obvious. The newer Watch arrived with a older version of WatchOS than your backed up Watch. You can't restore a backup from a newer WatchOS to an older one. In order to fix this, a slightly painful process is needed.
Set the replacement Watch up as a new Apple Watch.
As soon as it's completed the pairing and setup process, upgrade its firmware.
Once the upgrade has completed, unpair and erase it as in the first series of steps in this document. Awkwardly, that'll leave you with a backup of this "new" Watch setup. That's not a problem though: they're easy to delete.
After erasing the watch, when you go to pair it up once more, you'll see that your original Watch backup appears in the list of available Watch backups. Problem solved!
And yes, if you're curious, Reincubate iPhone Backup Extractor will let you see what exactly is in your Apple Watch backups. Your Watch backups are automatically shown in the app, although if you'd like to examine the data manually in expert mode, you should see
NanoBackup and its subfolders.