This article covers how to edit a Price Tier's Price, it assumes you already know how to create a Price Tier. If you don't know how to create a Price Tier check out How to Create a Price Tier before continuing.
1) Navigate to the Price Tiers page in Registration
2) Click the Edit button on the price tier you want to change.
3) Click inside of the "Price" box to change the price of your tier.
Note: You can't change the price of a Price Tier if there are registrations on it.
Also, if there are any active registrations (if anyone has been on the registration page) while you're trying to change the price of a tier you won't be able to change it. It takes 1 hour for any lingering registrations to expire. Then, you'll be able to change the price.