Select the delivery group you would like to edit by clicking "Edit" on the right of your screen.
After selecting the group you want to edit, you'll be given the option to update the delivery locations for the alias group. On the left side, you will see the group of addresses which this change will affect. On the right side, you will see the forwarding addresses you currently forward mail to.
To remove an address from that group, uncheck the checkbox next to the address under the "Will be delivered to" section. For example, in the image below, if you no longer wanted firstname.lastname@example.org to forward to email@example.com, you would uncheck the checkbox next to firstname.lastname@example.org.
To add an address to forward to, type in the new address in the text box. For example, in the image above, if you wanted to add a the forwarding address NewAddress@example.com, you would type it into the text box, then click on the "Add" button.
To finalize your changes, click on the "Update Delivery Addresses" button. This will update and save the delivery group you are currently editing.