Okay, so we have to separate tickets here. One person, flying on an award ticket, SEA-IST, and then continuing on another award ticket, IST-NBO.

The only thing to to here is to ask TK to change the IST-NBO ticket to a later flight. If there is no award availability, then this will need to be done via a ticket office, where they can ask HQ to release an award seat. Call centre will not do this.

(Because SEA-IST is a separate ticket and there have been no schedule changes, that ticket can only be amended as per usual rules.)