Go to the Bluetooth settings on your iPhone and check if the Apple Watch is "Connected". If it is, all you have to do is go to the Apple Watch app, then Apple Watch, unpair. The watch will take a few minutes to unpair. Make sure your Watch is connected to the power cable. When you pair it again, choose "Restore from Backup" and use the latest backup. If that one gives you problems and there's more than one backup, start the whole process again and choose the second newest backup.