To remove the last placed text block and restore anything on the fax that was covered by it, choose Undo Last. Undo Last works only once for each Place Text operation; you cannot undo more than one text placement.
Undo last only operates on the current page. For example, if you make changes on page 2 and then move to page 3 and press Undo Last, it will have no effect on page 2.
To undo all of the changes on the current page see Restore Page. Restore page works even when the Add Text dialog is open.