You can use an alias and then access with it in the array:
<?php
if ($_GET['action'] == 'Edit Student ID')
{
$c =$course;
$result = mysqli_query($mydbconn, "SELECT s2.UserName AS UserName
FROM student s1
INNER JOIN students s2
ON s1.UserName = s2.UserName
WHERE s1.CourseName = '$c'");
while(($row = mysqli_fetch_assoc($result)))
{
echo "<option value=\" " . $row['UserName'] . "\">". $row['UserName'] . "</option>";
}
}
?>
Hope this help.