How to unlock iPhone 7 Plus?

What kind of unlock?

• Activation locked? Log in to your Apple ID on the phone when setting it up, or log in to https://icloud.com, Click on Find then remove the phone from your account.
• Screen unlock? See this support article-->If you forgot the passcode for your iPhone, iPad, or iPod touch, or your device is disabled - Apple Support

If you meant something else, please enlighten us.

If you mean SIM unlocked, you must contact your carrier and ask what their unlocking policy is. All US carriers will unlock phones locked to their networks, and the instructions are on their website.

You should also remove Activation Lock; the instructions are in this Apple Link, which covers other steps that you should follow before selling the phone-->What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touch - Apple Support