Why can't I update my iPad to iOS 6 or 7?
Which version of the iPad do you have? Not every version is compatible with every version of iOS; in particular, the first-generation iPad cannot be upgraded to iOS 6+. If that's what you've got, 5.1.1 is the latest version your device supports.

See this table on EveryMac for details of which versions are compatible with which operating systems.