It does depend on what is meant. If the question is just "how do I unlock a phone from my carrier?" then the best way is to speak to the carrier and ask for an unlock code. There may be conditions, e.g. that you've had the contract for some length of time or you have paid off any phone subsidy, and if you are no longer a customer with them that will be a problem - you need to do these things while you are still with them.
If you mean "how to convert it to a carrier-unlocked model", i.e. without any manufacturer-specific software, then you need to be able to flash a new set of firmware to it. I do not know whether that's available/possible with this phone. You'd still need to unlock it as in the paragraph above before doing this (you don't want a modified phone that's still locked to the carrier).