If you are sure that you want to go to #1 instead, send #2 an email explaining that subsequent to their deadline you have been accepted to a program that is a better fit for you. Since you have not signed anything yet, I'm not sure if you have to ask to be released - maybe someone else can opine on that, but you should definitely email them before April 15. if #2 is a signatory of the Council of Graduate Schools' resolution and the offer you accepted was a funded offer, then your acceptance would be considered binding after April 15 and you would definitely need them to release you first (which they would probably do). Keep in mind that you should do this as politely as possible, since possible future colleagues at school #2 may be offended that you've changed our mind on deciding to go to #2.