iPad gen 1, iOS 5.1.1

Are you seeing an Activation Lock? You will need to contact the previous owner to have them log out of iCloud or go to https://www.icloud.com/find/ My Phone and erase the device, then remove it from their account. If you can’t do that, the device is useless. Apple won't unlock it.

Turn off Activation Lock - Apple Support.      

If that isn't the problem, what do you mean by can't update?

Do you have 2 Step Verification enabled on your account? There is a discussion of the Recovery Key in the article below.

Apple ID - Two Step Verifications FAQ    

You are welcome.