How do I free up space on my Apple Watch?

To Free-Up Storage

For songs, see:

Remove music from Apple Watch - Apple Support

and for pictures, see the "Limit photo storage on Apple Watch section in:

Choose a photo album and manage storage on Apple Watch - Apple Support

To delete apps on your watch, see:

Delete apps from your Apple Watch - Apple Support

Also, try unpairing and pairing your watch again and restoring from the backup just created in the unpairing process. This deletes all temp and corrupted files and re-indexes the file system. This should result in no loss of data and in some cases can free a lot of storage.

Unpair and erase your Apple Watch - Apple Support

Set up your Apple Watch - Apple Support

Also see:

If you don’t have enough space to update your Apple Watch - Apple Support