How to change a field in database through a submit button or Checkbox? [closed]

Well I would add another parameter to the change() function like id.
I am a little bit confused that your PHP code is wrapped in JavaScript Tags 😀

In PHP i would do it like that:

function change($id) {
    $sql="UPDATE wp_applications SET status="1" WHERE id = '$id'";
...

In the form i assume the id is stored in id column:

<input type="submit" name="activate" id="activate" value="Activate"       onclick="change($client->id)" />