Linking form responses by external user to existing record

Is there any way to link a form response from an external user to an already existing entry in a base? I would want the new info to override the old information (so if someone has a new phone number, they can add it and the old one gets removed)? I would not want their name to be editable though, so that we can link to the correct record.
I am managing a list of students for an after school music program. I’m using the “form” view to have students (external users) confirm enrollment for fall 2020 and provide updated contact information. I made the “first name” and “last name” column into single selects, thinking that it would link to their specific record, but the form creates a new entry.

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