You can change your rates any time via your profile settings - on this page. The change won't affect existing confirmed bookings.
If you change your rates, the new rates will only come into effect on future requests. If someone has made an enquiry with you but not yet booked, they will be able to book at the old rates. In order to be charged at your new rate, they will need to send you a new enquiry to obtain a fresh quote.
Please note that your rates need to be within a certain range, as they can't be too low or too high. If you choose a rate outside of this range, you will receive an info message.