In order to remove or add measures you will need to toggle on edit mode. When enabled the icon changes colors to orange.
After toggling edit mode on, click on any measure and follow the option you want. Insert bar will insert a new measure to the right of the one you have selected. Remove bar option will remove the bar you have highlighted.
If you want to remove more than one bar, you can select all the measures you want to delete and tap/click Remove.
If you would like to add measures to the end of a song, scroll to the end and click/tap and pull the tab staff. To add the measure, release when you see Release to add measure.
Don't forget to untoggle the edit mode once you're done. This will allow you to continue editing your song.