How can I upgrade from El Captain 10.11.6 to Mojave or Catalina?

The model and year of your Mac will determine which macOS you can upgrade to,

Please use Safari to access the following links do not use Firefox

How to upgrade to macOS Mojave - Apple Support

How to upgrade to macOS High Sierra - Apple Support

Before upgrading make a full back up of your Mac.

Check that third party apps you use have updates available for the new OS.

https://roaringapps.com

Check that your printers/ scanners etc have updated drivers and software available for the new OS.

Connect your mac to your router via cable rather than rely on WiFi.

Do not use your mac while it is downloading

and do not use any other devices that share the same internet connection.

If the App Store says Open then it is detecting a previous download of the install app

go to the Applications folder locate Install macOS ******.app.

Move it to the Trash, empty the Trash.

Restart, click on the link again, the App Store should say ‘Get' or 'Download'.

How to upgrade to macOS Catalina – Apple Support

If you are considering upgrading to Catalina be warned that it does not support 32bit apps.

Your Mac is probably running 32bit apps and these will need to be updated to 64bit apps,

some updates may be free, some may be paid. Use this app to discover which apps you have

are 32bit, https://www.stclairsoft.com/Go64/. Once the list has been created you can then

contact the developers of the apps to see if they provide 64bit updates. 

How to upgrade to macOS Catalina – Apple Support

On your version of macOS, the way to get the free update is through the App Store. Launch that and do a search for Catalina in the search field in upper left corner. It is a free upgrade.

Here is a support article with more information: https://www.apple.com/in/macos/how-to-upgrade/

Please be sure to make a full backup of your current hard drive. (Either with Time Machine, or a different service of your choosing.)

See Step 2 in these instructions for how to backup your Mac: https://support.apple.com/en-us/HT201475

On these How To Upgrade pages at Section 4 you click on Get macOS *****

and this should redirect you to the App Store, recently for some reason

Firefox has not been able to let users do this.

Be careful about going to Catalina.