we currently have three displays running in our company. Is there a solution to upgrade silently without the clients going blank or something like that?
The upgrade would be 1.7.0 to 1.7.5. Is it even necessary/recommended when we don´t have any (known)issues with the older release?
The answer you got from different forum is pretty accurate.
first you need to make backup database (to not lose media/layouts etc, see the cloning database page for that too) settings.php (to not lose well settings)
Then you replace the files in your existing installation with new downloaded version,
put the backup settings file back to your installation, then when you browse to your CMS url it should ask you to perform an upgrade.
Do not copy over the top means just that. Move the old files elsewhere, and then extract a fresh copy in their place.
The reason being sometimes we’ll delete files that are no longer required, and if you then leave your old versions in place, the CMS may not work properly because the old files will interfere.