No you cannot change the letter. Because usb is hotswappable, Meaning you dont have to turn your pc off and on again to use usb, It will change the letter every time, By default. Hope this helps bigg
If you intend to always leave it in the same port for backups, yes you can.
I do this and use drive letter U. (for USB)
Go to Control Panel-Administrative Tools-Computer Management-Disk Management-Right Click on your USB-Change Drive Letter and Paths.