Good question! Mistakes happen, even if you get work professionally done. You can remove glue from car window tint using rubbing alcohol, a common household staple that will make this job quick and easy.
You will also need a soft microfiber cloth and some warm, soapy water. Soap and water will help soften the glue, so it loses its adhesion to the tint. Then, dip the cloth into the rubbing alcohol and wipe the window tint until the glue loosens and comes off. Clean the entire window by pouring the leftover water over the glass, then rinse with clean water.
You can also use Goo Gone for a quicker solution, though its harsh chemicals may damage your tint or paint job, so be careful when using it.