SOFTWARE UPDATE

See Update your iPhone, iPad, or iPod touchhttps://support.apple.com/en-us/HT204204 and the related compatibility guides below.

iOS 13 and iPadOS are compatible with these devices - Apple Support.  

iOS 12 is compatible with these devices.  

IOS 11 is compatible with these devices (can no longer be updated to)  

To check out available apps and related updates, sign into the App and iTunes stores. Use the app icon on your phone.

What OS version are you using now and what year is your computer? Have your checked the App Store for updates?

Check the computer requirements shown in each link to see if your computer is capable of running that OS version.

Catalina

How to upgrade to macOS Mojave - Apple Support

How to get macOS High Sierra   

Sierra download

You can try to download it at the link below. If it won't let you, then your only options are to upgrade to Sierra/High Sierra/Mojave or find an older computer where you can download it using your Apple ID.

El Capitan

https://appleinsider.com/articles/19/11/23/how-to-get-an-old-macos-installer-from-apple