How can I manage nodes from 6am to 6pm thru SNMP, and un-manage nodes from 6pm to 6am?
What is the reason for unmanaging them? Just so that you don't get alerted on issues? If that's the case, change the time details on your alerts to not alert you between the hours of 6pm - 6am.
But to unmanage your nodes, you will need to do this manually each day.
On the SolarWinds server, you will find an 'Unmanage scheduler' utility that you can use to define recurring unmanage schedules