How do I fix horizontal lines on screen?

First, make sure you aren't accidentally booted into Safe Mode. When you are in Safe Mode some of the graphics will be reduced which could produce strange lines on the screen. If the issue is happening outside of Safe Mode, then you probably have a hardware issue. In most cases for these newer models, if you're seeing lines on the display it means that the display assembly is damaged. If you have AppleCare I would recommend contacting Apple to see if the repair is covered under your warranty. Otherwise you may need to consider replacing the display assembly. Good luck!