I don't believe you can return the vehicle. Always use the Tesla app to contact Tesla Service. You can attach up to (5) photos with the service request. I would take photos of the stains on the rear seat, other issues that can be photographed. List the issues with the vehicle at the time you brought it home. (I would even take a photo of the odometer so it is clear that the Model Y you purchased had less than 100 miles on the odometer at the time of the service request.) From the center screen inside the vehicle swipe right at the bottom of the screen just below the vehicle image. Then swipe down, down, down until you get to the odometer display.)
Don't attempt to clean the stains on the seat as you will probably damage the material. The seats covering is polyurethane material, requires little maintenance. Usually just water and a clean microfiber cloth is all that is needed to remove most stains. Then wipe dry with a second microfiber cloth to avoid streaking. You can also use unscented baby wipes when cleaning the seats. Don't use Magic Eraser as this product contains and abrasive and will damage the finish on the seat. There have even been reports of the adhesive glue used to tack down the seat covering to the foam padding of the seat causing stains. This type of stain should be a warranty repair.