Navigate to the event that contains the resource request you want to edit. Go to the "Resources" tab. All the requests for that day will appear. Note, that you can click on a room or item to view additional details such as setup notes and custom question responses. 

A pop up will appear where you can edit the details of the request. 

There are multiple ways to edit an individual resource request for a repeating event. 

The first way is to go to a specific day on the repeating event. This is the same as the steps in the section named "One Time Events" above.

The second way is from the listing of all requests for the repeating event. Navigate to the event and go to the "Resources" tab. Then click on the "Manage All Requests" button. A pop up will appear containing requests for all days in the repeating event. Search for the one you want to edit and click on the "View" button at the end of the row. This will open a pop up where you can edit the details of that day's request.

If you do not see the request in the table, make sure you select the right view (i.e. Past or Future) depending on whether the day the request has already passed or not.

Note, that editing only one request will not prompt to update in series because approval may be required.

In this case, the best option would be to delete the resource requests for the repeating event (which can be done in bulk), and then submit a new request with that missing resource added.