It all depends on what changes you want to make, but the standard way of altering forms in D6 (and D7) is invoking hook_form_alter() in a custom module. Do a switch on the form_id, and enter your edits, something like this:
YOUR_MODULE_form_alter(&$form, &$form_state, $form_id) {
// print $form_id; exit;
switch($form_id){
case 'event_node_form':
// change the form. The following will alter the text on the submit button:
$form['submit']['#value'] = 'Go!';
break;
}
return $form;
}
If you don't know what the form_id is of the form you want to edit, just uncomment the print $form_id
line and visit the form in your browser. You can refer to the D6 form api guide to find what form elements go with which properties. Hope that helps!