Hi @Kim866. @Gordon0 Is correct, that the guest can make the change themselves, and you then accept the change, or not.
If however, they don’t let you know, they arrived early, or they are unsure how to change the reservation, you can always do it yourself, but once again the date would need to be unblocked. You go into the details of their reservation. Scroll down and click on change reservation. Then click on change dates. Then make the date change to include the new earlier day, then when correct, send the details through to the guest. I wouldn’t be keen to leave it to chance for them to do the booking change. I’d want to clarify it and make the change. As @Alice-and-Jeff0 stated, you can always offer them a refund if they don’t get there that night...