iOS (ex. iPad or iPhone): 

On an iPhone X or later, or an iPad with iOS 12 or later, or iPadOS, from the Home screen, swipe up from the bottom of the screen and pause slightly in the middle of the screen. 

On iOS devices with a physical home button: Tap the home button, then swipe the app up and away to close it. 

See the following link for more information:

Android (ex. Google Pixel or Samsung Galaxy): 

Varies based on device, but generally, navigate to your device's multi-tasking screen, and swipe the app away to close it.

Windows (ex. Surface Tablets or Laptops): 

Select the "X" icon in the upper right corner of the application. This can also be accomplished by opening task manager and then ending the sales application as a process.