Log in to your profile and click "edit profile", scroll down to "Pricing" section at the bottom and change your price as required.
Your new price will only be applied to your future lesson slots. Past lesson slots and lessons that have already been booked (including those in the process of being booked) will remain at the original price.
If you only want to change your price for a specific client, you can do this by setting up a custom price
IF you're wondering what you should charge, you may want to check out this support article