How to unlock an iPhone 6 from Tmobile to AT&T? For free?

I found an iPhone 6 long time ago and I finally want to put it to use. But i don't know how to unlock it myself but I'm willing to learn and do it manually.

Just take it to an AT&T store. They'll get it going.

Well you can't unlock the phone yourself. You have to contact T-Mobile and send an unlock request.
Go here for more info: https://support.t-mobile.com/...s/DOC-1588

Just note: since you "found" the phone, if it's blacklisted, it can't be unlocked.

You must get T-Mobile to unlock the phone for you.

For free? I don't think so.

I got a metro store to do it for me for 10$. I found a T-Mobile iPhone on the street and took it to a metro store and they unlocked it.

Not going to happen. It was blacklisted when it was reported lost.