The easiest way to do it is by loading the new IOS into the primary switch flash and the stand-by witch flash. Make sure to set your boot variable, save your config and reboot both chassis.
This requires downtime and should be done during a maintenance/outage window and not during production hours.
Depending on your current IOS and the new IOS you want to run you can also use ISSU. see link:
http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst6500/ios/15-1SY/config_guide/sup2T/15_1_sy_swcg_2T/virtual_switching_systems.html#56355
HTH
