Hi
A letter of acceptance is an official letter issued to a student by the institution stating that he has met the admission criteria and is a student at their college/ university. ( it can be both conditional and unconditional)
an updated letter of acceptance would be a letter stating the last days after deadline for joining the college/ university, ( this again depends upon the institution)
A deferral letter is different from a letter of acceptance, a deferral letter is issued if a student wants to begin his study in the next coming semester rather than the one he is currently enrolled in .