Sometimes, when updating your Apple Watch, the update can seem to be running very slowly and taking a long time. Here's a look at how you can affect this and potentially speed things up.
Check your connection. If your internet connection is running slowly, the update can take longer to download. Stop any downloads running on other devices on the network to speed things up.
Switch off Bluetooth. On your iPhone, switch off Bluetooth to potentially increase the speeds of the update and data transfer.