Cand upgrade Old Macbook Air OS Mountain Lion

• I'm unable to log into iCloud on this old MacBook, I get the message 'you cant sign in now, try to sign in again'
• Tried downloading an update from the apple website but none of the links work. It just gets stuck on 'preparing download'

I've been at this for two days now with three computers. Its insanely frustrating. Any advice would help so much. Thanks!