Hi mpc120,
Thanks for using our products.
To upgrade from 3.1.2 to 6.0, you need to upgrade cluster to 4.x ==> 5.x ==> 6.0.x as show in support upgrade path in this document https://docs.couchbase.com/server/6.0/install/upgrade.html
To perform offline upgrade, you could follow the instruction on this page https://docs.couchbase.com/server/6.0/install/upgrade-offline.html
Since upgrade process relating to data management, you need to backup your data first before doing any upgrade. For cluster 3.x, instruction how to backup is here https://developer.couchbase.com/documentation/server/3.x/admin/Tasks/backup-cbbackup.html
If you have any question, drop me a line a thuan at couchbase dot com
Thanks