Apple Watch workout App
How do I reinstall the deleted workout app on my watch?
Apple Watch Series 9
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I can't see it listed in the App Store for a new download, so if it is urgent you might have to do it the hard way by unpairing then re-pairing and setup again.
Before you go to the trouble of unpairing I suggest you also balance the urgency to get it back, and the probability there will be a new software update available for your watch to reinstall everything. If you have not done a software update check now because WatchOS 10.0.2 is relatively new.