How to update ios to an older version?

I want to update my iphone 4.2.1 wich is unlocked to a carrier here in brasil, but the ios wich is on itunes 6.0.1 does not have a unlock yet, and i won't be able to use my phone, so i was wondering if i could update my iphone to an older version such as 5.0.1?

Added (1). So from what that first awnser said, i would have to first upgrade to 6.0.1 and then downgrade?

You can revert back to factory settings and then search online for the IOS updates or just update until you reach the one you want.

Alternative---…
http://www.idownloadblog.com/2012/09/28/how-to-downgrade-ios-6-to-ios-5-1-1-a4-devices/

Complete list of steps which will walk you through an easier(In my opinion) process

It would be called a downgrade, just so you know. As far as being able to do so, it wouldn't be possible without SHSH blobs.