The process of syncing your car's Bluetooth with your iPhone varies depending on the vehicle you drive. However on the phone it is generally the same.
All you need to do is...
Go to the home screen
If your phone is currently synced to a different system, tap on the system it is connected to in order to disconnect from it, and then choose the system that you want it to be synced to. In order to find out what you need to do specifically to your vehicle you may want to visit the manufacturer's website, or there are many forums that you can find online.