Call them.
Seriously, that’s always the easiest way to get any changes done with any airline. Czech Airlines does seem to allow passengers to login and see the status of of their ticket; some airlines also allow cancellations using their online system. I didn’t find a mention of this in their site help pages but you should give it a go. (You might already have.)
Otherwise, look up the contact details of the booking office most relevant to you. If you’re from Sweden, then details are:
CSA Czech Airlines
Kungsgatan 37
5 floor 111 56 Stockholm
+46 8 54 54 54 90
+46 8 58 53 67 69
Credit:stackoverflow.com‘
4 Mar, 2024
4 Mar, 2024
5 Mar, 2024
5 Mar, 2024