How to remove an old Apple ID from my iPhone 6s?

If you want to trade-in the phone, you’ll need access to the old AppleID so as to be able to remove the activation lock.

If you’ve forgotten the access credentials to that AppleID, see this:

If you forgot your Apple ID - Apple Support

Once you’ve regained access to your AppleID, you can then reset (erase) your phone. (ignore the section about “restore”)

If you forgot your iPhone passcode, use your Mac or PC to reset it - Apple Support

Finally, you’ll need to remove the device from the AppleID as described near the end of this article:

What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support

You can try resetting your phone with iTunes to clear your old Apple ID.

If you forgot your iPhone passcode -[Device] "unavailable" or "Security Lockout" on your iPhone...

If you forgot your iPhone passcode, use your Mac or PC to reset it - Apple Support

How to remove your iPhone, iPad, or iPod touch passcode without a computer

If you see a iPhone, iPad, or Apple Vision Pro unavailable message or Security Lockout screen - Apple Support

NancyRae63 Said:

"I need to change to change an old apple id on an 6s or reset the password so I have trade in the phone."

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Changing the Apple ID:

An Apple ID is no more than an eMail address. So, how many eMail addresses have you used in the past? And are any of them still open? If they are still open, then go here, seeing if you can reset the password: If you Forgot your Apple ID Password - Apple Support

Then...

Change it to What it Should Be:

Go Here: Change your Apple ID - Apple Support

Poor advice.

This will NOT clear the activation lock.