Your fee that you charge per lesson is entirely up to you and can be changed at any point.
The number shown on your profile to the students and potential tutees is your base price or display price. This is what all lessons will default to for a 1 hour booking. This can be changed by logging into Sherpa and heading to the 'My Profile' tab.
The fee you charge is not a set rate across all of your lessons. The fee attached to your profile at the time the booking is made will be the price that the bookings sticks to. Making changes to your fee does not affect the price of prior bookings already made.
You are able to alter your fee for an individual lesson by creating the lesson booking request yourself. When you create a booking, Sherpa will ask you how much you would like to charge for the booking. This does not occur on the student side so if you did have a booking that you wanted to charge more money for, for example a specialist subject, then you should create the booking request yourself.
When you create a recurring booking (for example a weekly session with a student), your rate will be locked in for that recurring session. You can easily check this by going to 'View and Edit' the booking, which will show the duration and rate for each session: