Step 1 is to talk with your HS counselor. They, along with you and your parents, will have to sign the second ED1 agreement. Directly work with the college you want to ED1 to now. Call the school and tell them what you want to do. They will likely tell you to apply EA (if they have it) or RD before the ED1 deadline and they will make the change to ED1 in their system. Then you, your parents, and HS counselor will have to sign another ED agreement before the ED1 deadline.
Good luck.