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')
];
}