When you upgrade to iPadOS the three dot menu will also let you close the slide over (or other apps)..
In the mean time, if you just swipe the three dots to the right it will move off screen. Swiping from the edge of the slide over also works. Swiping in from the right edge of the screen brings the slide-over back.
To get rid of any or all apps that are currently in slide over mode, swipe up from the bottom edge of the slide over. This will show you all the current apps in slide over. Just swipe each one up off the screen to close them.
Slide over is actually pretty useful. Say you've got something you want to keep an eye on, but not have to open and close your main app over and over. Like keeping an eye on an ongoing Message conversation, or Twitter. You place that app in slide over and you can just swipe in it in and out of view as needed, without closing what you are currently doing.
It is super handy when taking notes while in Safari. You can copy and drag stuff into the note, then swipe it back out of view so you can keep reading in full screen mode.