There is a new version of macOS out (at the time of writing 10.14 Mojave) and you want to upgrade. For the past few upgrades and this one Apple provided the update via the App store. This is all fine and well if you have a good unmetered(uncapped) internet connection to download the +- 5Gbyte installer. The big catch with the installer is if you follow the default install procedure the installer deletes itself after the install. This if of course a real problem if you want to update more than one Mac, especially if you pay per byte for your internet connection or if it is really slow.
You will find many articles on the web with workarounds for this problem. Unfortunately many of them are wrong, or even worse will provide links to untrusted installer downloads from third party sources. Luckily there is an official way from Apple to work arround this problem. You will find it at https://support.apple.com/en-gb/HT201372.
Basically you create a bootable USB disk by formatting it as “macOS extended” and then execute (copy/paste) the command from the support article mentioned above into the terminal.