Thanks for using our product.
If you want to upgrade OS, you need a new server, install Couchbase server 5.5 and do swap rebalance because each OS, we have a different package for it.
For example, if your server is ubuntu 14.04 and you want to upgrade to ubuntu 16.08, you need a new ubuntu 16.04 server to install Couchbase server 5.5 package for ubuntu 16.04. Then do swap rebalance to add new ubuntu 16.04 (data node) in and remove old ubuntu 14.04 (data node) out of cluster.
If you want to upgrade to ubuntu 20.04, you need to upgrade your cluster 5.5 to latest version like 6.6.
Here is the information of support OS of Couchbase server https://docs.couchbase.com/server/5.5/install/install-platforms.html
You could do the same step for other services.
If you have any question, drop me a line at thuan at couchbase dot com