Excel/Access update records

Good morning,

Im a little confused as to how to get this to work.
I have a userform that when filled in sends the data to MS Access to store as a database. (i have to use it this way as users dont have use of Access I can only use it to store data)

However, there is a time where the record thats been added needs to be changed.

How do I code a button to locate the record to be able to update? If the users are adding a unique reference everytime.

Ideally, I'd want the empty boxes to populate with what the record states so this can be changed..
(ps. Excel & Access 2007)

Thanks in advance.