A simple but important question (didn’t see an answering post yet) :
How much time on average does it take to the website to be updated when you modify files or settings (more like a quarter or 3 hours 
) ?
During this period, it won’t be offline, will it ?
Is there a command to instantly apply modifications (with no attention to people possibly on the site)
and by the way, is it gonna wait for people to disconnect or will it kick them out ?
How much time on average does it take to the website to be updated when you modify files or settings (more like a quarter or 3 hours
In normal cases instant. Please clear your browser cache and try again.
If your domain is linked with CloudFlare, then CF cache expires in 4 hours unless you login to CF and purge it.
Is there a command to instantly apply modifications (with no attention to people possibly on the site)
and by the way, is it gonna wait for people to disconnect or will it kick them out ?
What do you mean? All settings from 000webhost cPanel (except the ones from email forwarder/domain sections) are propagating instantly. Also, there is no need to kick out clients to apply changes to your site from File Manager/cPanel/phpMyAdmin.
Thanks for your quick answer I’ll try it (I always forget to clear the cache 
)
even if I don’t understand the difference : my website doesn’t have cookies and I never use the proposition in browser’s http line
Cache can store beside cookies, mixed content and requests. Your browser does this to prevent lag when browsing the site but when applying updates to host this feature is harmful as it does not always allow you to view the content in real time.
CF cache is the same thing, only it stores the content on CF servers.