New to Guitar Tab Creator is the ability to edit previously created staves.   Underneath the fretboard, within the tab controls, you will find a "Stave to Edit" select box containing a reference to each stave.  When you select a stave, the fretboard will load with the guitar tab you previously made, and you will be able to fully edit the stave of tablature, including the ability to add or edit timing.

The control looks as follows:

Upgrading Previously Saved Guitar Tab:

Only the currently editable stave of guitar tablature is editable going forward.  If you wish to take advantage of this feature, you will have to start over and recreate your guitar tab.