I'm surprised that no one has answered this one yet, it's pretty simple.

assuming that we are talking about windows and if you didnt find the answer yet.

you need to get to computer management. you can right click on my computer and then select manage or
click on start>control panel> administrative tools>computer management

once you are in computer management, on the left side click disk management. then on the right side look for the drive letter that you want to change, right click on it, and select change drive letter and paths. click change. click on the drop down menu and select the drive letter you want. click ok. click yes if it ask you if you are sure you want to do that. then your done, thats it.