In Notion for iOS, there is a beaming tool that can be found in the 7th pane from the right in the palette. The icon looks like two 8th notes with a beam trailing off to the right. If there is any question about which icon is the beaming tool, you will want to look at page 26 of the User Guide. The beaming tool is number 17 in that image. The User Guide can be accessed by tapping the icon of the "?" at the bottom of the screen.
To use the tool, simply tap the score where you would like to break the beam, and then tap the beaming tool.
