[chef] Upgrade Chef

[jfotop < >]

Hi to all,
I have a chef environment that I would like to upgrade.
Chef Server: 10.12.0
Chef-clients: 10.14.4 on CentOS 5 (and a few clients with 10.18.2 on CentOS 6)

1) Could/should I upgrade to chef server 11 and latest chef-client?
2) Is there any documentation on the upgrade process? 
3) Could I also upgrade ruby version to 1.9?

Thanks!