I'm using Joomla! 2.5, can I upgrade my website to Joomla! 3.0.x or Joomla! 3.x?
Yes, you can. Please make sure your system, extensions, template,... are Joomla 3.x compatible.
Then go to Administrator -> Component -> Joomla! Update -> Options -> Update server -> Update server: Short Term Support.
After making this change, you’ll only receive updates for Joomla 2.5. Click on Install the update to start.