Once a guitar tab object has been inserted, is there a way to go back and edit the tab?
If you mean the GuitarChord.ms object, you would select the marker for the object, which is shaped like a teardrop (it will be at the bottom of the chord chart) and press Alt-Enter or Ctrl-E. From there you can change any of the settings..
For help on what the various settings do, click the ? in the Properties title bar, and then click on one of the settings fields. You will see a popup explaining what that setting does.