Downgrade from iOS 6.1.3 to iOS 6.1.2 iPhone 4 / 3GS: Apple has patched evasi0n jailbreak exploits in iOS 6.1.3 firmware and the latest firmware iOS 6.13 can not be jailbreak by evasion jailbreak tool. iPhone 4 and 3GS users can perform a downgrade to iOS 6.1.2 which is still supported evasi0n jailbreak.
Downgrade from iOS 6.1.3 to iOS 6.1.2 iPhone 4 / 3GS
Things you need to download first;
Step 1: Connect your iDevice to the PC and open up Redsn0w 0.9.15 beta 3 and select Extras>Even More>Restore
Step 2: Now choose the IPSW button by clicking on it and browse to select the IPSW iOS 6.1.2 firmware file, which was downloaded in the first step.
Step 7: For those who rely on an unlock through ultrasn0w, click redsn0w and select prevent baseband update.
Step 8: Now the tool will ask to put the device in DFU mode; just follow on-screen instructions.
Step 9: Once in DFU mode, you’ll be redirected to previous screen.
Step 10: Choose the ‘local’ button by clicking on it and choose SHSH blobs for iOS 6.1.2 or if you want redsn0w to fetch SHSH blobs from Cydia, select ‘remote’, but make sure Cydia is holding SHSH blobs first.
Step 11: Click on Next once you’ve selected the SHSH blobs.
Step 12: Now let the tool downgrade the device to iOS 6.1.2, which will be done automatically.
Step 13: Once completed, the device is going to restart and will be on iOS 6.1.2 firmware.
After this you can easily jailbreak iOS 6.1.2 firmware with Evasi0n 1.4.