Rewards cannot be edited. This is so student data is not effected by editing the price of Rewards.
To change a reward, you can delete it and add a new reward item.
You'll still have access to data for the deleted reward. So if you're changing the price of a reward by deleting and re-adding it, consider changing the name so you can distinguish the old from the new in your data.
Simply click the red X on a reward to delete it from the menu.