Niš is located in south of Serbia, close to the border with Bulgaria and Macedonia. There is a highway between Belgrade and Nis.
Best option for traveling is via plane – Nis airport (check wizzair, ryanair, airserbia – try discount code BDAYAS2022).
Second best option is via Belgrade Airport – if you need assistance for traveling from Belgrade to Nis, write to the organizers (email@example.com). We will try to organize shuttle bus if more than 10 peoples are arriving at the same day.
You can also check nearby airports (not sure about bus connection with Nis):