This is the Retain chapter on doing the upgrade steps - https://www.novell.com/documentation/retain-4.11/install-upgrade/data/upgrade.html 

You would follow the "Upgrading from 4.9 to 4.11" section.  It is well documented and I follow this myself when I do upgrades.

You could also run Retain on a Windows Server if you lack Linux skills.  Here is the chapter on moving Retain from Linux server to Windows Server - https://www.novell.com/documentation/retain-4.11/install-upgrade/data/retain-to-new-server.html#mv-ret-lnx-win 

Hello, this is the GroupWise community. For this reason, I will move your question and the response to the Retain community. Thank you for your understanding.

What does the following mean?  If the index directory is using a sim link, another sim link needs to be created for the new index folder so the new index files are put on the proper storage.

This is instructions from the upgrade document regarding updating to the Solr 8 Indexing Engine., step 4

"sim link" = symbolic link.  It is a Linux file system concept.   Do an Internet search of "What is a Linux symbolic link".  You will get very good explanations of what that file system concept is.

I have not seen symbolic links used for any of the Retain systems I have worked with in the past 10 years.  Most customers allocate a separate large disk (i.e. /dev/sdb1 ) for the "/Retain" root directory used to host the Retain file system.

In the Retain upgrade to v4.11.0.1, the Solr 7 index directory is at /Retain/index/ and the new Solr 8 index directory is suggested to be /Retain/index8/ . The documentation is referring to that new directory at /Retain/index8/ .  You need to verify that the new Solr 8 index directory is where you want it so you do not run out of disk space.