In the RDS navigation pane, choose Instances, and then select the Instance that you want to modify
Choose Modify. The Modify DB Instance page appears
Under Database options, select the newly created option group under Option group
Click Continue to proceed to the next page
For the purposes of the demo, choose Apply Immediately so the changes are immediately applied to the Instance
Click on Modify DB Instance to Continue.
If you go back to the Instances Section, you would be able to see that the status of the Instace is Modifying, which should change back to Available once complete