1) Your change password form element has a space in the name, you cannot have spaces in the name of form elements:
<input type="text" name="Change Password" id="Change Password" />
change that to remove the space:
<input type="text" name="ChangePassword" id="ChangePassword" />
2) try adding the following at line 1 of the page:
<?php if(!session_id()) session_start(); ?>