Undo

The User Interface > Preview/Edit Window > Text/Edit Tab > Editor Toolbar... > Undo

You can undo any of the next group of buttons (Shift Left, Shift Right, Remove Line Breaks, Change Case, Text Clean-Up  Also, if you edit the text directly with the keyboard or mouse, you can also UNDO that as well.

The UNDO is one level only. For example, if you change the case and then remove the line-breaks, pressing UNDO will only UNDO the line-break removal.  If this button is greyed out, then there is nothing to undo.