I installed a kubernetes cluster running on CoreOs using CloudFormation a while back following this official guide: https://coreos.com/kubernetes/docs/latest/kubernetes-on-aws.html
Everything went smooth but now I want to upgrade my kubernetes version. Unfortunately I'm struggling on how to do this node by node and also on the master without taking down the cluster for maintenance.
I believe the CoreOS folks are still developing an upgrade strategy for kubernetes. Originally they shipped the kubelet in the base image but there are known update issues coupling the OS with the Kubernetes (https://github.com/coreos/bugs/issues/978). I believe the plan is to manage the kubelet via rkt (https://github.com/coreos/bugs/issues/1051) to allow decoupled updates of the two.
In the meantime your best bet may be to write some simple manual automation around the manual steps outlined in the CoreOS docs.