how to upgrade cluster build on rancher2.5.x on rancher2.6.0 ?

I install Rancher2.5.5(rke) and created cluster.
And I upgrade rancher to Rancher2.6.0 with helm upgrade command.

Rancher2.6.0 web page cluster management shows below.

State Name Version Provider
Active cluster-name v1.19.6 RKE

Pulldown menu Edit Config shows nothing.
I would like to upgrade cluster v1.20.x or later.
How can I upgrade cluster provided by RKE?

I’m sorry.
I used google chrome and chrome update failure shown.
I reinstalled google chrome and try cluster upgrade again, then upgrade success.