2
Normally you would just read the release notes and the directions for the release you want to use.
For 2.5.1 try these:
https://xoops.org/modules/news/article.php?storyid=5881You could also google 'upgrade xoops'
The most important thing to know is which version of Xoops you are using now.
You can find out by going to your root directory and go to include/version.php that will tell you what version you have.
Also which modules you have installed? Some old modules will not work with new Xoops.
If you list the modules the site uses maybe people here will tell you if they work and how to upgrade them, or which module can replace the old modules.
Welcome to Xoops,
John