Trainers manage their own rates and can change them at any time. Some trainers have different rates for group sessions, traveling outside of their travel radius, or because a client requests a discount. Trainers can write this sort of information in their profile, but are not required to. The best thing to do is to message a trainer with any questions you might have about their rates.
If a trainer agrees to change their hourly rate, you will receive an email notifying you of the change. In the event of an increase, you must agree to any changes a trainer makes to their hourly rate before a session occurs, which can be done easily through a link included in the notification email message.