How do I "unforget" a blue tooth device on iPhone 5s
I have forget this device with my car blue tooth device and now I do not know how to reset it please help
See the bottom of: Set up and use third-party Bluetooth accessories - Apple Support.
When you Forget this Device on your iPhone, iPad, or iPod touch, that accessory is removed from the list of available Bluetooth devices. To add the Bluetooth accessory to your device again, place it back in discovery mode and repeat the steps above to pair it again.
"It" in this case is the Bluetooth device you're trying to connect to. So you would need to check your car's owner's manual to see how to place its Bluetooth back in discovery mode.