Can't be done without modifying the files for the support manager plugin.
 
	In {blesta_dir}/plugins/support_manager/models/support_manager_tickets.php you can change lines 1230 - 1239 from
 
    public function getPriorities()
    {
        return [
            'emergency' => $this->_('SupportManagerTickets.priority.emergency'),
            'critical' => $this->_('SupportManagerTickets.priority.critical'),
            'high' => $this->_('SupportManagerTickets.priority.high'),
            'medium' => $this->_('SupportManagerTickets.priority.medium'),
            'low' => $this->_('SupportManagerTickets.priority.low')
        ];
    }
	to
 
    public function getPriorities()
    {
        return [
            'high' => $this->_('SupportManagerTickets.priority.high'),
            'medium' => $this->_('SupportManagerTickets.priority.medium'),
            'low' => $this->_('SupportManagerTickets.priority.low')
        ];
    }