In order to upgrade your mac to macOS Mojave you have to download and then install the full macOS Mojave
installer app.
The 10.14.1 disk image you downloaded probably from Apple Support Downloads is an update for macOS Mojave
you need to be running an older version of Mojave before that will install.
The next thing you need to know is that macOS Mojave version 10.14 no longer exists.
Apple removes all previous full installers from the App Store when a newer one is released.
The last full installer version for Mojave released was version 10.14.6 and this is what you need
in order to install a full working version of macOS Mojave.
Here is how to get it.
Open Safari and click on this link,
How to get old versions of macOS – Apple Support
Go to the Download OS section and click on macOS Mojave.
This redirects you to the App Store Mojave download page
it should say Get or Download.
Install macOS Mojave.app will be downloaded to your Applications folder,
the installer may launch automatically if not double-click to start the installation.
Before upgrading make a full back up of your Mac.
Check that third party apps you use have updates available for the new OS.
https://roaringapps.com
Check that your printers/ scanners etc have updated drivers and software available for the new OS.
Connect your mac to your router via cable rather than rely on WiFi.
Do not use your mac while it is downloading
and do not use any other devices that share the same internet connection.
If the App Store says ‘Open’ then it is detecting a previous download of the install app
go to the Applications folder locate Install macOSMojave.app.
Move it to the Trash, empty the Trash.
Restart, click on the link again, the App Store should say ‘Get' or 'Download'.
