I need help unlocking my iPhone 6s

As long as your phone is rejecting your passcode … the “Why and How” doesn’t matter … you’re NOT getting in.

You ONLY way forward is to reset the phone (completely erasing it) followed by a data restoration from a backup.

If you were using the default iOS backup and sync settings, you’ll find that the data restore will create a virtual carbon-copy of your phone as-of the date of backup.

It’s actually quite impressive how WELL it works.

If you forgot your passcode, you can use a Mac or PC to erase your iPhone. (This presumes that the iPhone is not Activation Locked.) Seems kind of a drastic fix ...

If you forgot your iPhone passcode - Apple Support

I am using the correct passcode but can not unlock my iphone6s. The only reason I think is that I recently bought a new android mobile phone and paired my iPhone to it via Bluetooth to try to transfer the data to the new phone. But I actually still want to continue to use my iPhone because I am familiar to it. The new phone is a backup. But now I suddenly can not unlock my iPhone. What can I do? I do not want to restore my iPhone because it will make me feel I am using a new phone. I just want my old iPhone back. I can use it as usual.

The thing is like your dog is old and you got a new puppy, are you going to put your old dog to death? That is why I hate Apple now.

Hating Apple will not make a security lockout go away.

Were you making backups of the phone before you got locked out? If you were, you could try to restore a backup after resetting the phone … although this might also restore the old passcode.