To update rewards for the employees to claim, first you need to navigate to the Rewards and Recognitions settings page. You do this by clicking settings in the navigation bar on the left hand side, then scrolling down until you see the area on the picture below.
Once you're on the rewards page, click the reward that you wish to update and it will take you to the update rewards page.
Here you can make the changes you need to make, including archiving the reward (by clicking the delete reward button). Once you're done, click the update reward button and your changes will be made live.