Hi
<blockquote>Will I lost any K2 and css modification that I made if I upgrade to the latest?</blockquote>
Regarding K2 the css will be untouched
While the template will overwrite some T3 files – anyway all your customizations can be stored in the custom.css file in this folder /templates/your_template/css/
The documentation about Ja_Extension Manager can be found HERE .
But first of all updates is strongly recommended to take a backup of all your customized files – so if you will rollback through ja_extension_manager you will not loose nothing.
Also through Ja_Extension manager you can check line for line all the changes made to the files –