First of all: If you got a Supersaver’s ticket (Sparbillet/Billet dégriffé), you can only refund it within the first 30 minutes of buying it. In that case, you can not get a partial refund. Otherwise:
If the ticket was already used and lies in the past: You can have up to one ticket per person partially refunded. You can only do this in person at an office at a station or by a local bus company, and only if it is no more than one month in the past. Source: Tarifbestimmung T600.9, section 1.7.2.1.
If the ticket has not yet been used and lies in the future, you can refund it completely in the app. Then you can buy the right ticket afterwards.