Yes, you can change your rates any time. The change will of course not affect existing bookings, but it will affect enquiries (message status "Contact").
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.
You can change your rates via your profile settings, or via these links:
If you do change your rates, as a courtesy, you should inform any clients who have enquiries with you.