How can I demotion from iOS8.3 to iOS8.1?

It seems that, there is no untethered for iOS8.3, so i want to demotion from iOS8.3 to iOS8.1,but failed.What should I do?

You mean downgrade, which is unavailable ATM.
You can either buy a 2nd-hand jailbroken iOS 8 device, or wait for the next jailbreak (which will probably come after the release of iOS 9)

OK, 3Q,It seems that, I have to wait for the next jailbreak.