I can't see any option to configure it but it says it's supported so I'd assume it's enabled by default. Next stop - ask TAC.
Or did you mean where to find the updated software/firmware? As always updates are in the Cisco software downloads:
I don't see any modem updates there yet so you're probably restricted to whatever is embedded in the latest IOS or TAC could provide updates to resolve specific issues.
The FAQ says "Bands in parentheses are targeted for future support with a modem firmware upgrade" so I guess they may plan to release the firmware in future or it may be be embedded in a future release of IOS-XE - which is how they're doing these firmware updates more often these days because it ensures the the version is tested to work with the IOS-XE version avoiding problems caused by mismatch between modem firmware version and IOS.
From what Cisco says, Carrier Aggregation is indeed enabled. but it does throw an incorrect message that its not enabled due to a bug and they are working on future releases to fix that. Carrier aggregation happens when the load increases either inbound or outbound when additional channels are added.