How can i change Fax Sender information on the header of the fax document such as 'From' and 'Fax' Number?

You cannot modify the "from" name unless you change the user extension's name from the service web. For number, you also need to login your account service web (service.ringcentral.com or service.devtest.ringcentral.com), select a user and click the "Outbound Calls/Faxs" and edit the Fax section accordingly.