Upgrading Gitlab from 8.4.2 to 9.2.5

We are currently on gitlab version 8.4.2 (omnibus)
our gitlab architecture is distributed as follows:

  1. main server - gitlab webserver and git server
  2. DB server - postgresql
  3. another DB server - postgresql replication from master
    4.redis server - caching

what is the procedure to upgrade distributed system like this one?

Thanks in advance