Hello !
May I please see the db structure (the table that needs to be updated) and a sample CSV ? The match by first + last name I'd say is not good, as there is a chance (slight, but still exists) that some wrong records might get updated, there needs to be some unique key used for identifying the csv rows against the table rows.
This is gonna be a PHP script that you'll have to run either in the command line or as a small web interface where you'll upload the file and process it.
Regards,
Dragos