To add measures to the end of the song, scroll to the end and click/tap and pull the tab staff. To add the measure, release when you see "release to add measures".
In order to remove measures you need to toggle edit mode. When enabled, you will also be able to add measures on a specific part of the song.
After toggling edit mode, click on any measure and follow the option you desire. "Insert bar" will insert a new measure to the right of the one you have selected.
If you want to remove more than 1 bar, you can also select all the measures you want to delete and press "Remove":
Don't forget to untoggle the edit mode once you're done. This will allow you to continue editing your song.