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Just for your info, you should always have a test site to check any changes that you make to a production site first.
As far as the upgrades go you will have to upgrade to 2.0.7x first then upgrade to the latest version.
Customizations depending on what they are, will need to be tracked so they can be restored (if still compatible with no changes).
Here is a document on
Howto upgrade from 2.0.7.3 to 2.0.9.2 with images and explanations.
Database and home directory backups always a good idea.
Any installed hacks (like auto login for instance) will need to be upgraded to a version compatible with the latest Xoops.
Also read all the faq sections pertaining to your situation, Lots of information in the faq. Read all readme files included with the upgrade files.
Any other questions or problems post here for help. And good luck with the upgrade.
HTH.