IOS – Is it possible to upgrade from ios 4.3.5 to 5.0.1 even though the current version is 5.1

Firstly, search Google for "download ios 4.2.1" and download the correct version for your device. If it's in a zip, unzip it, but you should end up with an .ipsw file.

Open iTunes and select your device on the left. Shift-Click (or Option-Click on Mac) on 'Restore' and select the file you downloaded.

This should allow you to install whichever version of ios you have the .ipsw for

You are using a version of redsn0w that cannot handle that specific version of iOS. You will need to find the correct version.

Here is a guide that might help: http://technorati.com/technology/gadgets/article/how-to-jailbreak-ios-435-on/

And it is of note that they have not released an untethered jailbreak for either 4.3.4 or 4.3.5, which means you will have to load the IPSW in redsn0w and apple the crack (select the option "just boot tethered") every time you cycle the power.

Also of note that the redsn0w .8b series is reserved for the iOS 5 betas.

You can see the full list of versions here: https://sites.google.com/a/iphone-dev.com/files/home/

Off hand I don't remember which will jailbreak 4.3.5 but give the latest .6 series a try first and work backwards.

And a word of caution, be wary of Redmond Pie, fromy experience they are far from a credible source for help and frequently report inaccurate information or often times completely spread misinformation. They seem more concerned with "getting there first" than actually reporting steps that you can rely on.