if I were doing this, I would upgrade to 2.0.10, then 2.2. You can find the downloads here: https://download.moodle.org/releases/legacy/
You can find instructions on moodle.org. You will want to read each of the upgrade documents in between before you upgrade the next step. Upgrading to 2.0.10 should be fairly straightforward--backup, replace the public html with the new version, copy in your config.php and plugins and log in as an admin to upgrade the database.
The next step, you should read the 2.1 and 2.2 upgrade:
Then:
- https://docs.moodle.org/23/en/Upgrading
- https://docs.moodle.org/24/en/Upgrading
- https://docs.moodle.org/25/en/Upgrading
- https://docs.moodle.org/26/en/Upgrading
- https://docs.moodle.org/27/en/Upgrading
- https://docs.moodle.org/28/en/Upgrading
- https://docs.moodle.org/29/en/Upgrading
The reason for reading each is that extra steps are sometimes required in an upgrade and while you can upgrade from 2.2.x to 2.9.x, you will need to do any of the extra steps required from all the versions in between.
Please make sure you back up all three parts of Moodle--the public html files, the database, and the data files (moodledata). And I'd seriously recommend installing a second instance on your server to test the upgrade in your environment before you attempt the live upgrade. Note: if an upgrade fails, restoring the backup is the only recourse, so make sure you back up first.
Upgrading definitely gets more complex the farther behind you get...