In Windows, it is quite easy to change the drive letters assigned to partitions. You can change the drive letter of external drives as well; Windows would remember the new drive letter the next time you plug in the same drive.
You must not change the drive letter of your primary partition.
Open Disk Management using Quick Access Menu
Open quick access menu by pressing WIN+X and choose disk management.
Open Disk Management through Computer Management (For Win7)
If you are using Windows 7 open computer management first.
WIN+R
Type compmgmt.msc
Hit ENTER
Choose Disk management from inside the Computer Management window.
Use Disk Management to Change drive letter
Right-click on the disk or partition you want to change and choose Change Drive Letter and Paths.
Click on the Change button to open the Change drive letter and paths Window; here you can change the drive letter.
After you click ok, you will see the warning message shown below.
Remember that even if you are relabelling a partition meant only for storing data, you might face some inconveniences if you are changing a drive label which has been associated with that partition for some time. It depends on whether you have set up any of your software to access that partition. For example, if you have set up your backup software to backup something from inside the partition in question, your next backup attempt will fail and you will need to adjust the folder paths in the backup software.
If you want to swap already assigned drive letters, you will have to it in three steps using one of the unassigned drive letters as a temporary drive letter.