How to install a previously downloaded IPSW file on iPhone directly without iTunes?
There's two problems here that prevents what you're trying to do:


OTA updates don't work by downloading the IPSW and installing it. They use delta updates, only downloading a 'patch' which is applied on top of the system.

You can download the patch yourself if you really want based on the version of iOS that you're currently on and what version you want to update to, but that's not going to help you, because…
OTA updates don't work on jailbroken devices. You'll end up in a boot loop and have to restore properly. Many jailbreak tools disable OTA updates for this very reason.


If your Internet connection fails whilst iTunes is downloading an IPSW, it will continue from where it left off when the connection is restored.