You have to do a jump upgrade form Joomla 2.5.22 to Joomla 3.3.x and from there, you'll be able to update from 3.3 to 3.4.8.
All is explained here on the official Joomla docs https://docs.joomla.org/Joomla_2.5_to_3.x_Step_by_Step_Migration ... read everything carefully and also, be sure to do a full backup of your website and database before proceeding. ;-)
Also remember that you will have to update all your third party extensions so they can be compatible with your new Joomla 3.4.x
You have to do a jump upgrade form Joomla 2.5.22 to Joomla 3.3.x and from there, you'll be able to update from 3.3 to 3.4.8.
All is explained here on the official Joomla docs https://docs.joomla.org/Joomla_2.5_to_3.x_Step_by_Step_Migration ... read everything carefully and also, be sure to do a full backup of your website and database before proceeding. ;-)
Also remember that you will have to update all your third party extensions so they can be compatible with your new Joomla 3.4.x
Last edited 19 February 2016 by Denis L.