How do you untighten a tight labret?

Try latex gloves or any other kind of disposable gloves that you would find in a first aid kit. Your piercer should have been wearing gloves when they put the jewellery in, and the gloves give a very tight grip on the metal, hence a labret stud that is difficult to unscrew.

Also ensure you use a bit of patience, and never pull on the stud or it could rip out.