how do I unlock my iPhone 6s

If it is Passcode Lock, check this link...

If you forgot your iPhone passcode - Apple Support

If it is Activation Lock, try this link...

How to remove Activation Lock - Apple Support

If your iPhone was owned by someone else, get them to unlock the iPhone, then turn off passcode in Touch ID & Passcode. You should probably also see about getting them to sign out of their Apple ID.

to unlock my other phone