There's no nice way to do this, but the most professional way is to call HR, thank them for organizing the interviews, apologize for the trouble and then say that your dream opportunity has come and you need to pursue it. Do not tell them which company it is. Try to keep the call as short as you can.

If you want be courteous, you could also call/email the hiring manager and say the same thing, but call HR first.

Also do it ASAP so that they can contact other candidates.

All the best and congratz for finding a better offer!