Red line on my display/screen

That sounds like a hardware issue. You should contact apple support for the best help, they can provide you with repair options.

Try these to determine if its hardware or software:

A.Take a screenshot (Sleep/Wake+Volume Up) and view the image in photos. If you zoom and the red line is there then it is a hardware issue.

B. If the red line appears while the device is on the boot screen too then it is a hardware issue.

What kind of red line is there on your display?

Does it always stay where it is? Because if so, I am afraid that there is a problem with your screen... There must be just a coincidence if this issue occurred at the same time as installing the new iOS.