You just need to edit the language file for JFBConnect. For English, that would be the /language/en_GB/en_GB.com_jfbconnect.ini file. Once in there, search for the string you're looking to replace.
You can also use the Language Manager of Joomla to create an override, which will be persistent even if you upgrade JFBConnect.
I hope that helps, but if you need more assistance, just let me know.
Go to Extensions -> Language Manager and then click on the "Overrides" tab. Click "New". Then, in the search box on the right search for "Register With". That will help you find the right string which you can then click on and edit the output language string on the left.
This was a really nice advice which I will certainly use it to several occasions but in this specific I cannot. No matter the way I wrote the register with I didn't have any results. I even tried with some spaces.
From where can I manually change it? Even if I have to repeat the steps after updates..
Are you saying the override wasn't working, or that you couldn't find the string to update in the override area? Just curious about that.
To fix directly though, edit the /language/en_GB/en_GB.com_jfbconnect.ini file (or whatever language you're using). The string you'll want to update is near the top:
Remove the words "Register With" on registration page
