Updating text field using checkbox
I currently have a Contacts table and a Companies table in my database. Each table has an Address, City, and State field. I already use my Companies table in a Value List drop-down field in my Contacts layout - so I can assign a Company per Contact. I am trying to also implement a checkbox field called 'SameAsCompany' that - when checked - will insert the Company Address, City, and State into my Contacts' Address, City, and State fields. How do I do that? Thanks.