Completely remove Field1 from your table.
Based on value of what actually you name Field2 you can do a computation in order to show in a form, report or query the "Out of date" message in an unbound text box or in a calculated field in the query.
Store this in your had:
If you can calculate something, do not store that "something" in your database.
If you do so, you will fully violate the main concept for the databases: the concept of NORMALIZATION.
Only for learning purpose
The answer to your question is: by using an Update Query.