It is not possible to refund Supersaver tickets. Nevertheless, there is an exception, if you bought a Supersaver ticket at half fare, while you do not hold the Half Fare Card. In this case, you can buy a new full fare Supersaver ticket and ask for a refund. A CHF 10 administration fee is applied, however.
You can find the refund request form here: Refund of tickets purchased online and via mobile | SBB help & contact
Alternatively, you can buy another half fare point-to-point ticket (not Supersaver ticket!). Show both tickets, the Supersaver half fare ticket and the point-to-point half fare ticket; they are together good for travel.
Finally, if you are going to Interlaken, you will probably want to ride the mountain trains and gondolas in the Jungfrau area. I strongly recommend considering the Swiss Half Fare Card, which could save you some money as they mountain trains are quite expensive. And you would not have to buy another ticket for Basel to Interlaken either