OS Mojave

The specs in the OP's signature indicate 10.13 (High Sierra) - so assuming that the complete Mojave installer downloads for him/her, it should be a straight update to Mojave. No bootable installer necessary.

However, in all cases - having a backup made prior to updating an OS is always helpful...

March76 Said:

"OS Mojave: Where can I find Mac OS Mojave to download? App store only has OS Catalina"

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Downgrading is Not an Option:

So, Try Using a USB Bootable Installer:

You would backup your Mac, install it anew, and then transfer everything this over from the backup.

Perform the Following.

I. Back Up your Mac First:

Create a Time Machine Backup, that way, you can have something to restore your Mac from, should anything go wrong with the install. Use an external hard drive that is at least 1.5 the total storage capacity as that of your internal hard drive. Once the new install is complete, you'd migrate it over. (See step 3)

II. Perform an Install:

Go Here: How to Create a Bootable Installer for macOS - Apple Support

III. Transfer Info From the Backup:

Use Migration Assistant to transfer everything from the backup you just made. Go here: How to Move your Content to a New Mac - Apple Support

This link will take you to an Apple Browser that opens your App Store with MacOS Mojave

https://apps.apple.com/us/app/macos-mojave/id1398502828?ls=1&mt=12

You are welcome. Thanks for clarifying.