How do I unlock an iPhone when the passcode is forgotten?

Hello!

The SOS in the corner simply means there’s a poor cellular signal, but you can still call emergency services if need be.

Please view the following to try and get back into the device: If you forgot your iPhone passcode or your iPhone is disabled - Apple Support

Have a great day!

If you niece has forgotten the passcode, she may need to reset the phone, wiping all data on it. During the process she may be asked for her Apple ID and Apple ID password.

After she regains access to the phone, she should carry out these steps to prepare it for reuse by you.

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

If it is left associated with her Apple Account, you may run into problems.