Please check the address that sent the email. Does it follow one of these formats?
XXX@XXX.apple.com OR XXX@apple.com
If so, the email is from apple and shouldn't be ignored.
If you think your Apple ID has been compromised - Apple Support
How is that possible?
There are numerous ways this could happen. For instance, you might have given your login details to a clever scammer, or used your Apple ID password for something other than Apple ID.
And how can I know the location of this iphone?
If the iPhone 6s is signed into your Apple ID, you can try this.
You should also enable two-factor authentication to increase the security of your Apple ID.