This removes changes you made on the image to the point as long as undo buffer is stored.
The following dialog will be displayed. Click either <<back button or next>> button, and choose the point you would like to get back to.

The setting of undo is in [file]-[Preference]. You can set the number of steps undo buffer stores. (Do not set the number of steps too high, or it will consume a hugh amount of disk space.)

> [file]-[Preference]-[undo]