Overwriting entry in table

Task:

To insert entries into a table. The table has a primary key based on a field 'ID'. When inserting into the destination table, I want to make sure that the new entry will overwrite the old entry.

What's the quickest/cleaniest way to do this ?

thanks,
ClaytonUPDATE <table>
SET <column>=<value>, etc...
WHERE field_id = <field_id>