iPhone 6 Plus screen is responding incorrectly

My iPhone 6 Plus does not correctly respond to my touch. For example, I would click on the U key on the keyboard and the Y key would be clicked instead. Also, I would be watching a youtube video and the screen would go crazy and rewind the video as if I was double tapping on the video to rewind 15 seconds. But I'm not clicking on the screen at all. I do not know anything about how to fix iPhone screens at all. I just want to know the possible causes. I am hoping I will be able to have this problem fixed if I brought it in to a shop or if I am able to fix it myself. Also if its any constellation, I bought the phone second hand from someone and so could it be possible that they had the screen replaced because it was cracked and the person who fixed it did not install it correctly? Thanks in advance :)