There is no mechanism for updating the check-in time of one reservation, @Brittany166 . Tell your guest that your guarantee that she can get in early is right in the messaging - your messages back and forth in the Airbnb inbox. She can point to those to prove she asked for an early check-in and you approved it.
UNLESS, of course, by checking in early you mean that she is coming a day or more early? If that is the case, go to the Change or Cancel button and follow the prompts to change the dates.