I accidentally returned a game and forgot to put the SD card in the box. I need to return the actual game, how can I do that?

It is best to get directly in contact with the library that the SD card belongs to. Assuming it was borrowed from one of the Helmet libraries you could contact their customer service through their website or bring the SD card to the library from which it was borrowed in person. To find the contact details for your own library, if it is part of the Helmet group, go to this website Libraries and services | Helmet and choose your library, which will give you the contact details for that particular location.