Hello Gideon.bina,
Thanks for posting your question! I'd want to make sure I was able to open my documents also, and I'll be happy to see how I can assist with this. I understand you're getting an alert about updating Numbers when you open some documents.
It sounds like there might be some underlying issues that could be relevant. When you say you're having some technical issues, what exactly is happening? Also, are you saying you upgraded from OS X El Capitan to macOS Sierra but you still see OS X El Capitan under Apple menu () > About This Mac?
Regarding Numbers specifically, it sounds like this only happens with some documents. Is that correct? Are you able to open documents successfully in other apps, or do you get a similar error? You might be able to test with Pages or Keynote.
Do you use iCloud? If so, you could try opening the iCloud website from your Mac and see if you're able to open one of the documents there. This could be helpful in narrowing things down.
Also, just to double-check, if you open the App Store and check Updates, are there any available?
I hope this helps you.
Kind regards.