Depends. If you open a camera raw file with the Photoshop camera raw plug-in and then you click the "Open image" button then the file open in Photoshop. If you want open the file again with Camera Raw simply repeat the initial process: normally in Photoshop go to File>open and select the raw file.
In Camera Raw, you can press the shift key, and that will change the "Open Image" button to an "Open Object" button. That will create a smart object that you can double click and reedit the original image, while keeping other layers that you may add - only the smart object will open in Camera Raw.