What software needs to be de-installed before downloading Mojave?

No software. However, if you are downgrading to Mojave, then you will have to erase the drive before installing it. After you download the Mojave installer from the App Store, you should make a bootable USB flash drive. Then boot from the flash drive which will open the Utility Menu. Use Disk Utility to erase the drive on which you will install Mojave.

https://itunes.apple.com/app/macos-mojave/id1398502828?mt=12

How to create a bootable installer for macOS

Boot Using OPTION key

1. Restart the computer.
2. Immediately, at or before the chime press and hold down the "OPTION" key.
3. Release the key when the Boot Manager screen appears.
4. Select the disk icon from which you want to boot.
5. Click on the arrow button under the drive icon.

1. Select Disk Utility from the Utility Menu and click on the Continue button.
2. When Disk Utility loads select the target drive (out-dented entry w/type and size info) from the Device list.
3. Click on the Erase button in Disk Utility's toolbar. A panel will drop down.
4. Set the partition scheme to GUID.
5. Set the Format type to APFS (SSDs only) or Mac OS Extended (Journaled.)
6. Provide a volume name, usually, "Macintosh HD."
7. Click on the Apply button, then wait for the Done button to activate and click on it.
8. Quit Disk Utility and return to the Utility Menu.
9. Select Install OS X and click on the Continue button.