Of course you can downgrade iOS 6.1.3 to iOS 5.0.1/5. But also you can use this new guide to unlock iOS 5.1 using UltraSn0w fixer 1.2.5.But first you need to jailbreak iOS 6.1.3 on your gadget. And one more thing: It’s recommended to do not install Ultrasn0w Fixer tool prior to the Dev Team’s Ultrasn0w being installed upon the iPhone.
UltraSn0w 1.2.8 can unlock iOS 6.1.3 on such basebands as:
How to unlock iOS 6.1.3 using UltraSn0w fixer 1.2.81. Launch Cydia and select the ‘Manage‘ tab from the tool bar at the bottom of the application. Select ‘Sources’ from the resultant list.
2. Tap the ‘Edit‘ option in the top right hand corner of the screen before selecting the ‘Add‘ button on the left hand side of the tool bar.
3. In the pop up alert view enter the repository URL which is:
then press ‘Add Source‘.
4. After the repository has refreshed and finished loading sources, tap on the repo name in the sources list which will display the Ultrasn0w Fixer utility.
5. Select the Fixer utility, pressing ‘Install‘ in the top right hand corner. Confirm the installation and then wait for the process to be completed.
6. Now that the Fixer package is complete, the current Ultrasn0w 1.2.8 software needs to be installed. Select the ‘Search’ option from the bottom of Cydia, typing in ‘Ultrasn0w‘and hitting the ‘Search‘ button.
7. When the Ultrasn0w tool appears, select it from the table and then select ‘Install’ as per step 5. After the installation has been confirmed and processed, the device should be unlocked as long as it meets the hardware and baseband requirements.
So now you see that it is working and have just unlocked iOS 6.1.3 using UltraSn0w fixer 1.2.8 on iPhone and can enjoy all features of latest Apple OS.