1) I agree with ISTFlyer: first see what self-service options, if any, TK offers you through the website or app.
2) If none or unsuitable for you, I'd try HUCA. TK phone agents are generally unempowered and uninterested in doing anything above the bare minimum (i.e., if they try something once and fail, they're likely to give up and tell you it can't be done), but you might luck into a slightly more helpful agent.
3) If that fails, fill out a feedback form on the TK website and wait for their offline customer relations team to help you.