To swap server sides either tap on the greyed out squash ball or fling the ball from one side towards the other.
The Squash Scorer app now has an optional “Smart Handout Side Selection” setting. When this setting is enabled, following a handout, the app will automatically start a player on their preferred serving side. The preferred serving side is the side they last started serving from.
The Squash Scorer app uses a toast message to indicate that a server change of sides was unexpected. This should make it obvious when this happens accidentally but wont stop it from happening intentionally to reflect the gameplay where it happens and isn’t stopped. Lower grades see quite a bit of this in my experience.