I'm new here. I would like to know how I can modify template. I was trying to do it directly from joomla but I have message that any change is forbidden
OT: may I write in English here or I should try to write in French?
I split the topic because the problem looks different.
If you try to modify in demo site, that's normal because user admindemo is not a super administrator and only a super admin can modify templates.
If the problem is different, can you post a screenshot of the message?
That's not the same problem.
If you try to use templates with a user that is not a super administrator, you will have a red message "only for super administrator".
In your case, it's not allowed to modify a default template (for security, to have a template which works).
You have to create a new one.