JetBlue Airways doesn't have a dedicated physical sales or customer service office in Hannover, Germany you'll need to book/manage flights online at jetblue.com, use their 24/7 call center (potentially with fees for agency bookings) at 1-866-829-1086, or contact your travel agent for assistance with bookings made through them, as they serve Germany with flights to cities like Berlin (TXL) and Hamburg (HAM) but focus on online/phone support.
