I am unable to update my iPad to a more recent update, possibly 13.0 if that is most recent. My iPad is old but still works. I am unable to download apps

See If you can’t download or update apps on your iPhone or other Apple device https://support.apple.com/en-us/HT207165.

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Also, see See Update your iPhone, iPad, or iPod touch

https://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 (Cannot be updated to anymore.)  

But, if you are having trouble updating,

See If your iPhone, iPad or iPod touch won’t update

Also 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