Hi Sergiosp,
May I know which JoomlArt extension are you wanting to update? I will check it for you.
Pls note that if you're mentioning JA Builder template, this is a special template from us and at this moment, you can't update via JA Extension manager unfortunately. You can edit a page created via JA Builder, if there is new version available, there will be a popup appears and you can decide to update or not. We're updating JA Builder so it can be a standalone package and would be update via JAEM too.
Regards