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Hello and welcome to XOOPS forums
AFAIK, you have to follow the upgrade path for each conquetive version untill you reach the current version release.
Of course the number one step is to BACKUP everything first
Then reade any readmes in the patch files.
it usually requires (and this is only a generalised comment) that you overwrite existing files with new files, by copying new files into the appropriate folders on the server.
Then going to your admin panal and updating the system module and any other modules that have been installed in the site.
If you have any hacks in your site, you need to back these up also as some newer files may overwrite them.
To update without consequetive upgrade paths would require you to compare each previous upgrade version to identify any files that occurred in one realease, but not another and include these to the final version files that always had an update in each release. In other words if header.php is found to always have an update in each version, you could possibly skip to the latest release and likewise any other file that was ALWAYS updated.
It would be complicated process, better go through the staged process, unless you get a better advice here
If it were my site, I would make a copy in a sub-dir and go through a practice run first, its up to you.