Iphone 6

You cannot go back to earlier versions of iOS and the only upgrade option will always be the newest version of iOS appropriate for your phone model.

It's not possible, and even if it was, you would find that a number of apps would not work as they increasingly require iOS11 as a minimum

Rafa2007 wrote:

Good day!

I want to upgrade to firmware 10.3.3 again because my phone worked stably with it. How can i do this? I tried through the iTunes program, but it does not allow me to switch to this firmware.

That's not an upgrade, it's a downgrade which is not supported.

iPhone 6 can update to iOS 12.4.4 that's it.

Can you read?

Downgrade to iOS 10.3.3 is NOT supported.

And there is NO way to do so. Apple doesn't support downgrading, you will have to stay on 12.4.4.

That's why iTunes won't let you restore 10.3.3, because it is no longer being signed. Apple controls which firmware can be installed.