How I unlocked my iPhone 6s Plus to any brand?

Assuming you mean it is SIM locked, see this Apple support article—>How to unlock your iPhone for use with a different carrier - Apple Support

If you mean unlock from a carrier to a different carrier, you have to contact the carrier the phone is locked to and ask if you qualify for an unlock. ONLY the carrier a phone is locked to can unlock it.

The article Lawrence provided tells you you need to contact the carrier the phone is locked to.