Any unwanted editing or alterations which have been carried out in the Layout Window can be reversed simply by undoing them. This can be done either by going to Edit Menu > Undo or using the shortcut ⌘Z. Several steps can be reversed quickly by using this command over and over again. If you go back too far, the reversed steps can be re-done either by going to Edit Menu > Redo or selecting the shortcut ⌘⇧Z.
If you are not sure whether to undo a certain step, it can be useful to go to the Edit Menu as the name of the last function carried out is recorded next to the Undo command.
The number of steps available to undo relies on the setting in the History window. Go to Window Menu > History > Set Maximum to find out how many history steps you have set. Alter this number if you require more or less actions to be remembered.
Note: The History window itself does not record every action carried out in the Layout window. It only records colour changes made to the design. This is because the history only records changes which actually affect the design information, such as changes to the pixel information or layer colours. Therefore, use the Undo function to undo changes to your colourway layout and use the History to undo colour changes.