When upgrading any software it's usually a good idea to check the manual first, and read something like this:
To upgrade an existing MongoDB deployment to 3.6, you must be running
a 3.4-series release.
To upgrade from a version earlier than the 3.4-series, you must
successively upgrade major releases until you have upgraded to
3.4-series. For example, if you are running a 3.2-series, you must upgrade first to 3.4 before you can upgrade to 3.6.
Since your current version is pre-3.4, you'll follow the documentation rabbit hole until you reach the page that tells you how to upgrade from 2.6 -- et voilà, your entire upgrade path lies before you in all its glory.
I guess you'll want to install all the required MongoDB versions on your target server simultaneously and open your database, starting each one in sequence, following each version's upgrade instructions.