How to INSERT INTO a specific row?

I have a form with a calendar at the top, below it is a list of items each with a check box.

Here is a wonderful visual of it -

Calendar

Checkbox | Col | Col | Col

Checkbox | Col | Col | Col

Checkbox | Col | Col | Col

I want it so you select the date and it gets passed into the database in the same row as the one (definitely one, multiple is better) that has been checked by the user.

I was trying an INSERT INTO with a WHERE statement but apparently you can't do that. So if I can use a WHERE with INSERT INTO, how can I identify which row to insert the info?

Currently it is passing the dates but it creates a new row.

I didn't think posting the code is neccisary for this question but if anyone wants to see it let me know and I will post it.

THANKS!