How to Restart your Samsung Galaxy Watch 4
Press and hold the Home key on the side of the watch and swipe down from the top of the screen to the quick panel. Tap the power icon to turn off the watch.
To force restart (reboot) your Samsung Galaxy Watch, press and hold the Home and Back keys together for about at least 10 seconds until the screen goes black, then release. You should see a message that the watch is rebooting.
Source : myhealthyapple.com