How to remove an icloud lock from iPhone 5s?

Okay, so, I sold My iPhone on Amazon and I had the buyer notify me that it is locked. How do I remove that with no iPhone on me? I'm pretty clueless when it comes to iOS since I use Android. If there are various methods please let me know.

Invert colors stuck as default?

I've got an iPhone and the invert colors are stuck as default (when I turn on invert colors, it goes back to normal) how do I fix this?

Is there anyway you can delete a Facebook message?

I sent a message to someone and regret sending it but the person hasn't seen it yet. Is there a way where I can just delete the message that way that person doesn't see it. I know I can just swipe on my iPhone to delete it but the person that I sent it will still be able to see it once she sees it

How long should I use my Smartphone?

I got a iphone 6 which my mom owned for over 2 years. I cleaned it up, and am reqlly happy to have it. The performance is great and the 16gb memory is fairly enough for me. What do you think, I don't think that I'm gonna smash my phone soon. How long should I own it? (I'm 16)