How do disconnect a phone call from my Apple Watch?

If you want to dump the call simply hit the red button on the watch screen to hang up. If you want to move the call to a different device you need your iPhone. Look for the phone call, a green bar at the top of the screen and tap it to display the phone app, then hit the speaker button on screen to display the choice of available options.

Answer phone calls on Apple Watch - Apple Support

Just tap the red phone icon on your watch face...or, just cover your watch with your hand.