If you are referring to upgrading in place, in general:
- Follow RedHat instructions for upgrading OS in place:
- Install el7 CM/agent/daemon packages on the host after upgraded
- ensure el7 parcels are available for download/distribution.
Depending on your needs, you may want to decommission the host so that blocks are copied to another node (HDFS). I would recommend that if you are undertaking this procedure, you plan well and also do a test by adding a node or testing with one as a trial. There are bound to be small issues in various places.
I've never done it myself, so if anyone else has experience, it would be good to hear from them.