how to upgrade Big Sur to mojave

The move from Big Sur to Mojave will be a downgrade.

If your Mac is new and came with Big Sur installed, it cannot be downgraded.

If your Mac came with Catalina or earlier, you can wipe the drive and reinstall the previous OS.

You will have to create a bootable installer for Catalina.

How to create a bootable installer for macOS - Apple Support:

https://support.apple.com/en-us/HT201372

• EDIT - Reinstall On an Intel-based Mac: If you use Shift-Option-Command-R during startup, you're offered the macOS that came with your Mac, or the closest version still available. If you use Option-Command-R during startup, in most cases you're offered the latest macOS that is compatible with your Mac. Otherwise you're offered the macOS that came with your Mac, or the closest version still available.

How to reinstall macOS - Apple Support:

https://support.apple.com/en-us/HT204904

Big Sur is newer than Mojave. As such the upgrade would be from Mojave to Big Sur, not the other way around.

What exactly are you trying to accomplish? Do you actually need Mojave for something? Otherwise, its best to stay on Big Sur.

MacOS Big Sur 11.5 is a newer OS than macOS Mojave 10.14.

That would be a downgrade and downgrading is a lengthy laborious process

and you may find that any documents or files you created or edited in Big Sur

will no longer open or be useable in macOS Mojave

This will also be the case with the Photos app and iTunes in Mojave and

iMovie if you use those apps.