Docs link:
http://docs.mongodb.org/manual/release-notes/2.6-upgrade/#upgrade-the-cluster-s-meta-data

To upgrade the meta data, run:

mongos --configdb <config servers> --upgrade

You can include the --logpath option to output the log messages to a file instead of the standard output.

1. It would be nice to change <config servers> to <configDB string>, and clarify that the configDB string should be the same configDB string used to start the mongos, with identical host names and ports listed in identical order.
2. Clarify that it is also required to include the options that used to start the mongos. For example, if the --sslOnNormalPorts --sslPEMKeyFile options are used to start the mongos, it is still required to include these options when running mongos --upgrade to upgrade the meta data.