Jag hittade denna info i deras FAQ, tyvärr framgår det inte om det tillkommer avgift utöver telefonkostnaden för detta:

Thank you for contacting Ryanair.

If you have made an error when entering your passport details you will need to contact your local call centre in order to make any changes, please click on the link below for contact details.

https://www.ryanair.com/en/questions/contacting-customer-service/

Please note that if you have any further queries relating to this request that you will have to submit another request via our webform.

Yours sincerely,

The Customer Service team

For and on Behalf of RYANAIR LIMITED

Det går att ändra det mesta inklusive destination, namn och avgångstid. Men det kostar ofta förmodligen mera än att köpa ny biljett:
https://www.ryanair.com/se/resevillkor/tableofoptionalfees-tableoffees/