How do I close open apps

Open Apps can be force-closed using the App Switcher…

To expose the App Switcher, it is necessary to swipe upwards with your finger from below the active portion of the screen, to the middle of the screen - and pause briefly - before lifting your finger. Once visible, open Apps can then be closed by swiping the corresponding App thumbnail off the top of the screen.

Alternatively, the App Switcher can be summoned by a double-click of the Home button on iPads so equipped.

Ok

if your on a iPad 9gen or Lower click the home button twice then you swipe up on the open apps if your on a pro swipe up from the bar below the screen here is some info

Quit and reopen an app on iPad – Apple Support (AU)