If you're already using the cluster in production and require no downtime you should perform a datacenter migration first to change the RF to 3.
Rough process would be as follows:
Change keyspace to NetworkTopologyStrategy with RF=1. You shouldn't increase RF here as you will receive read failures as not all nodes have the data they own. You would have to wait for a repair to complete to stop any read failures.
Configure your clients to use a LOCAL_* consistency and DCAwareRoundRobinPolicy for load balancing (with the current DC configured)
Add a new datacenter, configure it's replication to be 3.
Rebuild the new datacenter by running nodetool rebuild <old-dc> on each node in the new DC.
Migrate your clients to use the new datacenter, by switching the contact points to nodes in the new DC and the load balancing policy DC to the new DC
At this point you could increase the replication factor on the old DC to 3, and then run a repair. Once the repair successfully completes you should have 2 DCs that you can use. If you need the DCs in separate locations you could change this step to adding another DC in the desired other location and running rebuilds as per steps 2-4.