Call and ask to speak to a supervisor.
But before you do that, do your homework and search (extensively if necessary) to come up with some preferred alternatives on your own. Then when you call and speak with a supervisor, see if they'll give you what you ask for. The key is to have all the details of your own preferred alternatives at hand, so they don't have to do a lot of searching for you.
Airline staff can, if they're willing and - critically important - if they have been empowered by their management to solve customer issues - bend or even break some rules.
When you buy a flight ticket, you and the seller are entering into a contract. The terms of the contract are pretty explicit: you agree to pay them, (and to behave...no skiplagging, etc.). They agree to transport you from Point A to Point B on a specified date and time (within limits). A few details are supposed to be unchangeable: the origin and destination cities can't be changed (though not the stops in between) and flight departure date can't be changed.
So going by the contract, they can't fly you to Point C or from Point X instead, or depart a day earlier or later. But if you ask nicely, and you ask the right person (and maybe give them a reason) they MAY bend the rules and allow you to change one or more of those things. You can always get a refund if the alternatives they offer are not acceptable to you. The new flight might cost more than what you paid, but they might wave the fare difference. They may refuse to do anything and just offer an unattractive alternative, no negotiations (and technically, the departure and arrival cities are supposed to be set in stone...so you should know going in that changing that is a big ask).
When a flight changes, it's not necessarily a bad thing. I've had a flight change that allowed me to negotiate for a nonstop flight (which would cost more) than my original (cheaper) 1-stop or 2-stop itinerary. So a flight change can (sometimes) get you a much better flight, maybe at no extra cost. But you need to ask. And they have to be willing to play along.
Note: Once you accept the alternative flight that the airline's computer has offered you, then your negotiating position is much weaker, so whatever you do, don't just click OK on the airline's website when it says your flight has changed, here's what we're gonna put you on. Do your homework, summon all the charm you can, and call them to see if they'll make it right for you. Good luck.