How do I account for the check box in list pages?

I added a button to the list page. Admin can choose people via the check box. When pressed, the button needs to change the GroupID from 1 to 6 for all checked records. Here is the code:
// Put your code here.

//if ($_SESSION['GroupID'] == 1)

$sql="SELECT UserID, GroupID FROM UserValidation ORDER by UserID DESC LIMIT 1";

$rs=CustomQuery($sql);

$RecData=db_fetch_array($rs);
global $conn,$strUserValidation;

$strSQLSave = "UPDATE UserValidation SET ('GroupID') values (";

$strSQLSave .= "6";

$strSQLSave .= ")";

db_exec($strSQLSave,$conn);
However, how do I account for the checkboxes in the list page? As I said before, people click the check boxes and then the code above needs to change them all. How do I do this? Do I use the WHERE command? If so, what is the variable name for the checkboxes? This is the HTML for the checkbox: <INPUT type=checkbox {$checkbox_attrs}> Is this the variable? How would I format this in the code above?
Mike