Yes, the major companies (reluctantly) should let you unlock if you are out of a payment plan for the hardware. This was something they fought in court. It's nice AT&T lets you do this yourself, but even if you have to visit a store for them to do it, you are entitled when you own the hardware.
If any method refers to "jailbreaking" your phone, that is both technically complex and risky, in the sense of no guarantee of success, no guarantee against side effects, and no assurance as to the safety of the tools you are using to make it less technically challenging.
You could also just buy your phone upfront, not through a provider, and it will come "free" already. Then just get a SIM card.