2. Generally, to downgrade, there must be a downgraded version of the local SHSH file (the SHSH of other people's equipment cannot be borrowed), and the extraction of the local SHSH must be carried out in the case of jailbreak. IOS8.4.1 has not released a jailbreak tool, so it is impossible to extract the local SHSH file.
3. About SHSH files: the full name of SHSH is Signature HaSH blobs, which is Chinese signature hash, also known as shsh blobs. Shsh is a signing certificate generated by Apple authentication server according to the ECID and firmware version of iOS device. When we use iTunes to restore firmware, iTunes will submit the ECID of the restored firmware device to the Apple Verification Server and apply for a certificate of restoration verification. Once verified, the Apple server will send a signature certificate corresponding to the ECID and firmware version to iTunes, so that iTunes can continue the firmware recovery of the version corresponding to this shsh.
4. The premise of the big version downgrade is that the current IOS system also needs jailbreak, but IOS9.2.1 has not released jailbreak so far.
5. Even if there is a degraded version of SHSH file, the success rate of degradation is not high in jailbreak.
6. Give up decisively.