How do I upgrade an iPad-1/iOS 5.1 to iOS7

I have both, a new iPad Air, on iOS7 ( all works fine); the 2nd is an iPad - 1, on System 5.1.:

- I would like ro upgrade ( using iTunes to System iOS7, (or at least iOS6), as I use it sometimes as well;

- When I connect the iPad-1 to my Mac I can update ( & sync) my photos ( in iPhoto), also

Apps etc, but iTunes does not offer me to upgrade .

1/ Is there a possibility ( Service mode ??) to force iTunes (under. devices) To offer me the upgrade

window and

2/ what do I have to do, to avoid data loss or corrupting the iPad ?

( BTW, I gave successfully configured my new iPad Air using " Restore from backup".

What sequence of actions is required ?

Kind thanks in advance,

Best, hs