Overview

The upgrade of Mongo can be performed in several ways, but in general there are 2 ways:

Upgrade binaries

This method involves upgrading to binaries from one version to the other. It can be very exhaustive and repetative, if you are staring from old release.

Migrate to newer version

This method is way better if you are staring from old release (e.g. 2.6). In nutshell, you will perform a logical export and import into the newer release. However there are some limitations to that method.