Depending on the phone model you are using:
Go to the bottom of your screen, then:
- Either click on the menu button (three lines or a square) then swipe between the open windows (left / right) and then swipe up over the window you'd like to close.
- Or if your device doesn't have a visual button for this, swipe up from the bottom left or right side of your device until you come to an overview of your recently used apps. Then swipe between the open apps (left / right) and then swipe up over the window you'd like to close.
Please read more directly here: https://support.google.com/android/answer/9079646?hl=en