To change a price on a registration, go to Edit This Campaign > Manage Registration > Edit the Registration Group. Here, scroll down until you see the section "Registration Types" and change the price and market value as needed. (Price - Market Value = Tax-deductible amount).
If people have already registered for this registration type, you will not be able to make pricing updates directly to this type. This is to keep data integrity in the database for those who already registered.
Here is how to make those updates for registrations that have already been booked:
Create a new registration type with the correct details, and then "Hide from the public" the registration type that is incorrect.