You are probably going to have to go to the airport to make the change.
As for the fees, that's Ryanair for you. That's why the fares are cheap. But it can get interesting. If you checkin online, you don't pay to checkin, but if you checkin at the airport, you have to pay a fee (5 euros IIRC) because you are using an airport agent. However, if you are not a passport holder in Europe, you cannot checkin online. So me being from the US, I would have no choice but to pay a total of 10 Euros roundtrip just to get checked in for the flight.
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