Click or press Ctrl+Z when is selected, or select Raster Edit Menu > Undo.

Undo undoes up to 256 raster edits.

You cannot undo the following raster edits. When you use them, any pending raster undos will be lost:

Rotate by Degrees ().

Rotate by Line ().

Auto Deskew ().

Scale.

Border.

Clear buffers.

Reduce color depth.

Increase color depth.

Pending raster undos will also be lost when you vectorize the image.