How do I update my property's star rating on my Google Business Profile?

Google gathers data for class ratings from a variety of sources, including third-party partners, direct research, feedback from hoteliers, and machine learning inference that examines and evaluates hotel attributes, such as price, location, room size, and amenities. 

To change your star rating, you must contact Google support. The instructions can be found here: https://support.google.com/business/answer/7660515?hl=en