Working in Pixelmator, a change that you do not like or did not mean to make can easily be undone.
To undo a previous action choose Edit > Undo.
To redo an action that you have undone choose Edit > Redo
The Undo command is used quite often, so you might want to memorize the Command-Z keyboard shortcut for it.
SEE ALSO
Each time you save an image version, a copy of the previously saved version can automatically be retained. That way, if you change your mind about a particular edit that you made, you can return (Edit > Revert Document) to the backup versions of the image.