How to find the letter for the usb connected

The easiest way would be to open Windows Explorer and navigate to My Computer (well, your computer, but you know what I mean). If the phone is connected as a drive it will show there, and if it did not connect as a drive but as another kind of device it will probably show there.

If it does not, you can look for it in Device Mangler or, if any software came with the phone, install said software.