If you saved complete SHSH for 6.1.2 some iDevices can be upgrades.
Advise of iDevice model and if you saved SHSH
I never once upgraded to iOS 6.1.2, so, no, I don't have SHSH blobs saved. Well, my iDevice is a bit outdated. I'm still using the iPhone 4. My phone doesn't support Siri. My iOS that came pre-installed was iOS 4.2.10. It's a bit outdated but it still does me well.
Anyway, I'm looking for instructions like this one from one of my other threads that darricksailo wrote.
Download ifaith and choose the options to dump your blobs. The program will guide you through the steps
1. run ifaith
2. click build signed ipsw with blobs
3. select the dumped .ifaith extension blob (keep a local backup of it just in case)
4. it'll verify that the blob is valid and complete, then choose download for me when it asks you to choose the firmware
5. once done downloading, depending on the instructions, choose the downloaded ipsw and click continue
6. it'll build the ipsw package and stitch the blobs in it
7. once it is done building the ipsw package, it'll give you instructions to put the device into pwned dfu mode
8. once in pwned dfu mode, open itunes and hold shift, then click restore
9. select the custom firmware that ifaith made for you and it'll restore to iOS 6.1
10. once on iOS 6.1, download evasi0n and jailbreak