Singapore Airlines (SIA) doesn't seem to have a physical, dedicated office in Zagreb, Croatia instead, for bookings, changes, or support, you'll need to use their online channels or contact their regional/global support, with some unofficial listings pointing to a potential third-party or general contact number, but the official Singapore Airlines Help Centre or phone support (like UK's +1-866-829-1086 for UK/Europe) are your best bet for official service. Jauntlet.comhttps://jauntlet.com/