How to fix iphone 6s plus screen shadows?

So i just got an iPhone 6s plus 64 GB and its amazing but there are shadows of the bottom part of the home screen on other apps its very faint and also i can only see it on dark themed apps but its a little annoying can some one help? P.S (its pre owned if that helps)

What is wrong with my IPhone? - 1

I have a 5s, and it has acted weird forever. It's run out of service randomly, not sended texts etc etc. Those problems are all gone, but now there's a new issue. Whenever I go to camera roll, it says "IPhone Storage Almosg Full. You can stop this by deleting unneeded apps, photos, videos, and music." So then I listened to this message by deleting every single ******* photo, video, and app i had, until the only thing that I had left was the undeletable apps. I also tried resetting my settings, and that worked, but it fixed everything besides that camera thing. I need someone to tell me how to fix this.

Can I just upgrade my iPhone 6 to an iPhone 8?

I'm already on a family plan, I can backup my memory on the cloud. Can the Verizon SIM card in my 6 be used in the 8? Can I just keep everything as is, stay in the plan. Basically all I want to do is buy the iPhone 8 as I've already replaced the battery for the 6 once… And it's not holding power well anymore.