Oh okay so you're familiar with the fastboot flash method. Then yeah, same steps, just no wipe haha. Sorry. I went way too far with that guide I guessoh duh so just the same as flashing but dont factory reset at the end. i'm a boob.
another thought, without twrp, are there any nandroid/twrp backup type options? just cause ive had bad experiences with dirty flashes before
And I'm the last person to ask about backup solutions, sorry :\ I never back anything up or have anything worth backing up so I never have this issue. Sorry I can't help here.