Apple Watch
I mistakenly deleted the apple fitness app from my Apple Watch. How can I get it back?
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First try to download & install the Fitness app from the App Store.
Download apps and games on your iPhone or iPad - Apple Support
This worked for another user with essentially the same problem Unpair and pair your watch again and restore from the backup just created. This should result in no loss of data. It deletes all temp and corrupted files and re-indexes the file system.
Unpair and erase your Apple Watch - Apple Support
Set up your Apple Watch - Apple Support
See:
How do I reload the workout app on my wat… - Apple Community