Best to contact ÖBB on twitter (if you have it) or facebook, instagram.
They might be able to help you and change it.

You’ll most likely to need to buy a new one as reservations are not normally changeable.

If it was just a normal seat-only optional RES then just think of it as ´learning curve spent´. Make a new one if you think its really needed for 2nd.

If it was something else, check first condiitons etc-maybe there is even some partly hidden way to change it, but often ,IF possible, only for other day/same trip and not so much for things like this

Thank you everyone! I have made a second reservation in second class :)