how do i put onchange in


$regionslistx[]= JHTML::_('select.option', "", JText::_( '--No Location--' ),'value','text' )

or


$lists['regionslist' ]= JHTML::_('select.genericlist',  $regionslistx, 'City', '', 'value', 'text', $lx )

Within the second situation, where you will find the empty argument, put the function you want to use:

$lists['regionslist' ]= JHTML::_('select.genericlist',  $regionslistx, 'City', 'onchange=\"YOURFUNCTION()\"', 'value', 'text', $lx );

Also look into the documentation: http://api.joomla.org/Joomla-Framework/HTML/JHTMLSelect.html