How to delete apps from my watch, so I can update it

Remove an app from Apple Watch

• Grid view: On the Home Screen, touch and hold the app icon until you see an X on the icon, then tap the X to remove the app from your Apple Watch. It remains on your paired iPhone, unless you delete it there, too.
• List view: Swipe the app left, then tap  to remove it from your Apple Watch. It remains on your paired iPhone, unless you delete it there, too.

Note: Not all builtin apps can be deleted.

Try this;

Unpair your Apple Watch via the Watch app on your iPhone.

Then Re-pair your Apple Watch again,this usually frees up

enough storage space to update your watch.And without any loss of data.