If you did not use overrides, just like the frontend, your edits will be lost. It is the exact the same method but instead of the frontend template you add the overrides to the isis (what a terrible name) template.

I'm afraid I don't know in which file I can put this override.

And this site is under Joomla 2.5, so the template is Bluestork.

It's the exact same method.
You need to do what you normally do with eg: mod_k2_content with /administrator/modules/mod_k2_quickicons.

I think I already did this kind of override, but I don't remember how. Il will study that later.

Thanks for your help Krikor.

This post will help you :)
getk2.org/documentation/tutorials/174-templating-with-k2-and-the-concepts-of-sub-templates