Lineups are always subject to change as explained in the Terms and Conditions agreed upon purchase. As follows:
10.3 The promoter reserves the right to alter or vary the programme of an event without being obliged to refund or exchange tickets
Sadly, you are not entitled to a refund should the headlining artist(s) change from the one listed, or if another artist(s) advertised to perform, drops out.
Why not go to the event anyway? You might find out you like it - Nirvana once replaced Pearl Jam in Seattle…