upgrading macOS El Capitan

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

please use Safari to access the following links,

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 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 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

Catalina. It has the updated security and stuff. But still depends on the compatibility of the device. Refer to this link on how you can upgrade to catalina: https://support.apple.com/en-us/HT201475

The problem is that I can't upgrade to Catalina 15.4 unless I already have Catalina 15 installed and I cannot find macOS Catalina 15 anywhere in the apple store.... i can find it outside of apple but then it is not a trusted developer and won't load....