I upped the pixelation of image size in the Image menu, saved, and closed the program. After reopening the same document I realized that more tweaking is needed but can't do so properly because even after lowering the pixelation down to its former amount the image doesn't go back to its former state. Can't revert the image either. Is there another method for restoring the default settings?
You do have Time Machine and a few days of backups are kept on your primary APFS drive. Turn on Time Machine in Apple System Preferences and add its icon to the menu bar. From there you go to the Folder where you kept the file, and from the TM menu bar icon, choose "Enter Time Machine" and go back before you edited the file. There you can restore the prior version.