Lineups are subject to change as agreed upon purchase.
Generally, 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. This is especially the case for festivals as you have paid for an event where lots of artists are performing. If the headline artist for an event drops out and isn’t replaced then refunds are at the discretion of the event organiser.
Recently, we have seen a huge surge in artists having to pull out of events, which as you can appreciate is outside of theirs and the event organisers control during this difficult and volatile time. If you no longer wish to attend due to a change in line up then you can pass your ticket(s) on to someone else.