Go home, press the upper button, tap the activity you want to use, swipe up and tap Settings, swipe to get to "Auto Lap". In that row, you'll see "Auto Lap" and (e.g.) "5.00 mi" on the left, then a vertical bar in the middle, then a vertical "toggle switch" on the right. Tap the "toggle switch" on the right. It'll be filled green with the toggle at the top if it's on, and filled grey with the toggle at the bottom if it's off.
