I have iOS 7.1.2 and I want to update to iOS 8.4.1 but it says i can only update to iOS 9.
I don't want 9 because it would work very bad on my iPhone. How do i upgrade it to the right version that I want?
How do I update to iOS 8.4.1 and not iOS 9?
You can't - Apple is no longer signing older versions.
The exception of course is when an older device is restored to it's original shipping version.
If you want a device at an older iOS, you need to find one that has been fossilized.