I believe Apple is still signing 5.1.1 signatures, while it's still being signed you can upgrade to 5.1.1 via iTunes. First you would need the IPSW for 5.1.1 which you can find here on iClarified.
iPad firmware download: www.iclarified.com/8500/where-to-download-ipad-firmware-files-from
Once you have 5.1.1 firmware
1. Open iTunes
2. For Mac; click option/restore in itunes and select 5.1.1 firmware you just downloaded and confirm restore.
For Win; right click on restore button, select the 5.1.1 firmware you just downloaded and confirm restore.
Hope this helps you out and remember Apple will stop signing 5.1.1 signatures soon. Once they stop signing you will not be able to upgrade to 5.1.1