K3s fails to upgrade inside rancher 2.5.x

I have a standalone installation of Rancher on dockers, using the following command:

docker run -d --restart=unless-stopped --privileged -p 80:80 -p 443:443 -v /opt/rancher:/var/lib/rancher --name rancher_2_5_7 rancher/rancher:v2.5.7

I tried to update the local k3s using the UI menu, but it failed. now it displays a message looping on and off

This cluster is currently Upgrading.
Cluster is being upgraded

on the Edit Cluster menu, shows “Kubernetes Version v1.20.4+k3s1”, but accessing directly to the container and issuing k3s --version, shows “k3s version v1.18.8+k3s1 (6b595318)

since it can not be downgraded there is nothing to do trough the UI, I tried to update k3s directly inside the container replacing the binary and using the curl installation script without success. either fails to connect to k3s or the container terminates

Can be k3s updated directly inside the container to correspond to the rancher’s UI displayed version?
Or should I modify some file inside /var/lib/rancher in order to get versions to match?

Everything else is working correctly, upgrading other RKE cluster works as expected