This should work:
"UPDATE ".TBL_AGENT."
JOIN ".TBL_SUBSCRIPTION." ON ".TBL_AGENT.".agent_id = ".TBL_SUBSCRIPTION.".agent_id
SET ".TBL_AGENT.".agent_name = '$_POST[agent_name]' , ".TBL_AGENT.".company_name = '$_POST[company_name]' , ".TBL_AGENT.".email = '$_POST[email]', ".TBL_SUBSCRIPTION.".renewal_date = ( UNIX_TIMESTAMP() + ( '$_POST[subscription_renewal]' * 86400 ) )
WHERE ".TBL_AGENT.".agent_id = '{$_GET['id']}';"