Well, you have most likely chosen the college you will be attending next fall. That’s great.
If you are lucky enough to have been in the position to have turned down some offers, be thankful- and gracious, to those schools you will not be attending.
Obviously, those you had to say “No thank you” to are great schools. After all, you chose to apply. And the admissions officials did a lot of work on your behalf: reading your application and essay, considering your future and your compatibility at their school. Someone cared about you and considered you a great candidate. So, thank them. Send a personal letter and address it to the admissions official who signed the acceptance letter to you. Let him or her know that you appreciate the consideration. And if the school has a survey, fill it in and honestly answer why you chose another school.
Who knows? Maybe a school you declined will be in your future. You may choose grad school there, or want to transfer. It is not only ensuring yourself a plan B, it is the right thing to do. You appreciated all the kind letters from the colleges that accepted you- be kind back.
All the best!