It says iOS 11 is no longer supported. How do I upgrade it?

You will need an Iphone 6S to update to the latest version of IOS. It seems you have not updated your phone for a long time if you are stuck on IOS 11.

Go into Settings > General > Software Update

This is where your phone checks for updates. If you can upgrade to 13.5.1, great that's the latest IOS but you might not be able to depending on your device even if you are not running a supported version.

Tip: Back-up your phone before an update. Although things are very unlikely to go wrong, it's always useful to have a backup.

For more help please check the link below but otherwise Thankyou for using Apple Forums r5151 and I hope you have a wonderful afternoon!

Update your iPhone, iPad or iPod touch – Apple Support

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    If you can upgrade to IOS 11, you can upgrade to IOS 12

Update your iOS device.        

Get help with over-the-air iOS updates          

Update – Can’t or can’t restore 

Have you tried using iTunes on a computer or Finder if running Catalina to do the update?

 Update software on iOS devices in iTunes on Mac

If you see an error when you update or restore your iPhone, iPad, or iPod      

Go to Settings -> General -> Software Update.

If there is an update for your iPad it will be there.

It is possible your iPad is not able to be upgraded to iOS 11 - for example, if it is an iPad 4, iOS 10 is the latest version of iOS supported by the iPad 4.