you find all upgrades under http://sourceforge.net/projects/xoops ... 0%28stable%20releases%29/
If you want to be uptodate you should use xoops 188.8.131.52
As there have been a few bigger steps in xoops from 2.3 to 2.5 I think, you could have several problems especially with older modules.
I would recommend:
- to set up a new xoops
- install then module by module (check first for newer versions of each module)
- if the complete system and all modules working proper then import the content data of your modules
of course, this is more work than updating step by step, but at the end you have a clean new system.
Additional, if more then one module is not compatible with new xoops version, it is easier to find out which one.