Navigate to the Team settings by clicking on your name on the top right and selecting 'Team settings' from the menu that appears. Then click on the 'Rewards' tab. From here you can create new Rewards or edit the existing ones.
When adding or editing a Reward, you can modify the name, image and description. You can also decide to enable/disable this reward or modify which groups of people can give this reward.
Be sure to 'Save' your changes once you have finished and take a look at how to Give Rewards if you are unsure.