How can I update my Mid 2012 MacBook Air to version 10.14?

Start your upgrade project here: How to get old versions of macOS - Apple Support, and—as mentioned—use Safari to download the installer. Have a complete backup created before the upgrade, too. That backup is your path to recovery, should something go wrong.

Safe Mode is slower. However, if you want to try downloading it again in normal mode, feel free. I usually recommend Safe Mode because some have mistakenly installed third-party antivirus/security apps which can interfere with the download.

This link takes you to 10.14.6 how can we get 10.14.0? Also when i click the link to open, it says "This copy of the Install macOS Mojave application is damaged, and can‘t be used to install macOS."

Why do they make it so difficult to update and find older versions?

There is no 10.14.0, Apple removes its older full installers when newer ones are released.

Therefore the only full installer available linked to earlier will install a full running version of Mojave 10.14.6.

If the App Store says open and gives you the error message about it being damaged the resolution to that

was discussed earlier in this thread.

JordanSpeig wrote:

This link takes you to 10.14.6 how can we get 10.14.0?

You don’t. Not unless you saved a kit. The App Store download is updated, which means it’ll be on the most recent update of that macOS version, save for minor changes.

Apple doesn’t require folks to download all of the .0 version and the most recent combo kit and any patches to the most recent update.

Also when i click the link to open, it says "This copy of the Install macOS Mojave application is damaged, and can‘t be used to install macOS."

If the delete-and-redownload fails as has been suggested, set your system time to manual and set your date to September 30th, 2018, launch the installer, then when you get to the first prompt defer responding, switch back to System Preferences, and re-set your system time to automatic.

Why do they make it so difficult to update and find older versions?

Running older versions inherently gets progressively more difficult, and folks that need and want to do that will get to learn a whole lot about preparation including caching old kits, and knowing how to use them should an embedded certificate expire, as well as dealing with the inevitable and degrading network connectivity. (Connectivity issues are not a problem with 10.14 as yet, but that’s been arising with 10.11 and 10.10 and earlier as the embedded security ages out.) This as the rest of the internet upgrades their security. And newer macOS versions check in with Apple when installed.

Running old versions gets progressively harder. Used to be somewhat easier too, before everything was networked to everything. Now, not so much.

Open in Safari: macOS Mojave 10.14

Delete "Install macOS Mojave" from the Applications folder. Try again while in Safe Mode.

How to use safe mode on your Mac - Apple Support

Once it's installed.

You're welcome.