Just tap on a score to add a point. It's easiest to do this after each point when you're getting ready for the next one. As is common, the server's score is on top, so after a game it will switch to the bottom when the receiver becomes the server. To make it easy to remember, your opponent's points have a cowling image around them, and your points are clear.
If you hit the wrong score, or hit a score more than once, long press on the watch to bring up the Undo Menu. If you hit Undo, it will revert back to the previous point.
At the bottom is the current score in games. Tap this bottom score to see a full summary of games and sets won.
Tap on the game score to see a summary of all scores.