Whenever I lower its brightness, lock, & unlock my phone, flickering occurs. How can I fix that?
Probably the easiest way is to try to restore your device. Back it up first though, then wipe it and restore it. If it's a software issue, it'll get resolved. If it doesn't get fixed, you're most likely dealing with a faulty screen.
If it is a faulty screen as well I would NOT recommend replacing it yourself. I tried this and ruined my last phone. If it is an option, just get it replaced or have it fixed by a pro.
Most flickering issues have to do with the phone being knocked or dropped. I have never seen it where it was just from software issues. This is not to say that can't be the issue, I just never seen it myself.