Why is my Joomla! 2.5 site update manager showing an update to 3.x?
From Joomla! Documentation
Not unless you’re 1000% sure. By default, Joomla will not let you update to 3.0 unless you activate the option within the Joomla! Update component (administrator >> Component >> Joomla! Update >> Options >> Update server >> Short Term Support). If for some reason you’ve changed this, and you’d like the updater to stop letting you upgrade, change the setting to Long Term Support. After making this change, you’ll only receive updates for Joomla 2.5.
