You need to include CBs API, once done you'll need to establish the $user object; you can then adjust a users field data or user data as you feel necessary. If wanting to change only 1 field I recommend just using queries as loading the full CB API is not logical just to adjust 1 field.
Example as follows (assumes CB API is loaded!).
[code:1]
global $_CB_framework;
$user_id = $_CB_framework->myId();
$cbUser =& CBuser::getInstance( $user_id );
if ( ! $cbUser ) {
$cbUser =& CBuser::getInstance( null );
}
$user =& $cbUser->getUserData();
$user->cb_field1 = 'TEST';
$user->store();
[/code:1]
For further information please see tutorials in signature.