sir, for my iPad 2 I am currently on version 5.1. I wish to manually update it in sequence to ios 6 followed ios 7; please help on how todo it

Dear Sir, I am currently on version 5.1 and wish to upgrade to current iOS version in sequence. I need to update to iOS 6 then to iOS 7 followed by upgrade to current version. I tried doing upgrade manually but I do not see any way to do it manually using iTunes. Could you please detail the steps for manual update process. There are several sites where older versions are available but how I should apply those using iTunes. Please help.