How can update my iPhone 4

That is not possible. The iPhone 4 does not have sufficient hardware to run iOS 8.

If you need to run iOS 8 then you are going to have to replace your obsolete iPhone 4 with a newer one that can run it.

Connect you phone to iTunes on your computer and update.

hi

how i phone 4 update 8,.0 ios

You don't. That model can not be upgraded beyond 7.1.2