how can I download mac OS Mojave 10.r to upgrade my MAC

If you have done all the preparation and are ready to upgrade your Mac you can use the link on this page to acquire the installer.

Use Safari to click on the link under the heading Download macOS using Safari on your Mac which will find the page for you in the App store.

The Age of this computer will Govern what newer version of macOS it will Qualify to install. Use About this Mac from the Apple Icon on Desktop - the required Information will be there.

With the above information in-hand - follow this How to get old versions of macOS and verify what version this computer Qualifies to install. Make sure to use Only Safari to commence the download as Others may not work. Safari will open a redirect and open the Apple Store where you can commence the download. If an error message appears, that is probably because one is using another Browser.

Make a Time Machine Backup Before attempting. Notation - Mojave is the Last version to support 32 bit applications / extensions and Drivers.

Are there any AntiVirus, Disk Cleaner, Optimizers, Un-installers, etc installed which should be removed as per Developers Instructions. They are useless, unneeded, cause havoc and interfere with the normal operation of the OS and may even Corrupt the OS requiring a Reinstallation

Best Results are always over an Ethernet Cable to the Router.

Wifi is too slow and tends to have packet losses. That causes a Corruption of the Full Installer.

If Wifi is Only Option - move computer as close to the Access Point / Router and disconnect all other devices connected to Wifi. Want the maximum bandwidth for the computer 

Do not attempt doing the download via Connection sharing and using Cellphone Data as means of Internet Connection.