Action List

The action list remembers what you've done to your image and lets you revert back to any earlier version of the image.

The action list is located middle of the right side.

To display the action list, choose View > Window > Action.

Action List

Every time you select, paint, and resize etc, each of those states is listed seperately 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.

Options

To specify the maximum size of undo, choose View > Option > Document. For more detail, see Option > Document.