My software update 13.5.l on my IPad will not download, what do I do?

See Update your iPhone, iPad, or iPod touch

https://support.apple.com/en-us/HT204204  and the related compatibility guide below.

iOS 13 and iPadOS are compatible with these devices - Apple Support. Your phone fits in here and allows the latest version of iOs 13. 

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And, if you can't update, see:

If you see an error when you update or restore your iPhone, iPad, or iPod https://support.apple.com/en-us/HT201210  and

iOS update and restore errors https://support.apple.com/en-us/HT204770

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Also, you might be experiencing a server problem. See If you can't connect to the App Store, iTunes Store, or other Apple services https://support.apple.com/en-us/HT201400. There have been quite a few Apple server connection problems lately. Among other things, in the above document, see Check Apple’s System Status page. You can check there to see if the needed server is available when you need to use one. 

Get help with over-the-air iOS updates          

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

Use this search link IPad will not download. Maybe you can find something I am not aware of.