Every time you select, paint, and resize etc, each of those states is listed separated in the action list. To revert to a previous state of the image, click the name of the state on the action list or press Ctrl + z.
Tips
Hornil StylePix keeps track of up to 64MBytes of your last actions. But you can change the size of undo level by adjust the undo level size slider.
To specify the maximum size of undo, choose View > Option > Document.