Keep your Apple Watch paired with your iPhone
Back up the paired iPhone to iCloud or to a computer
Make sure the iPhone backup completes successfully
Unpair the Apple Watch from the iPhone to trigger an automatic watch backup
Keep the iPhone and Apple Watch near each other during unpairing
Wait for the unpairing process to finish before setting up the watch again
Restore the Apple Watch by pairing it with the same iPhone and choosing the backup during setup
If using a new iPhone, back up the old iPhone first, then restore that backup to the new iPhone before pairing the watch