Hi becca_753,
From the support article Update your Apple Watch:
"Get ready to update
Next, check these things:
- Make sure that your iPhone is updated to the latest version of iOS.
- Make sure that your Apple Watch is at least 50 percent charged.
- Make sure that your iPhone is connected to Wi-Fi.
Keep your iPhone near your Apple Watch throughout the update. It could take from several minutes to an hour for the update to complete, so you might want to update overnight or wait until you have time."
Ensure that your Apple Watch remains on the charger and is connected to Wi-Fi during the update process to allow it to complete. If your network is slower or has security measures which affect that connection it could take longer or not be possible to update. In such cases attempting to update on a different connection may be necessary.
Regards.