Didoune, I have a number of suggestions, not all of which are exactly about your use of English - some concern researching who you are writing to in order to more accurately address your letter. You may, for example, be writing to the academic registrar at the institution - you may even be able to find out their name online. The polite thing to do when writing is to consider the needs of the reader.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.