So lets began with the method.
1- Open cmd as "Run as administrator" option
2- Type Diskpart and press enter.
3- Type List volume and press enter.The no. of partitions of your hard disk will be shown on your screen.
4- Choose the drive whose letter drive letter you want to change and type "Select volume #" where # is the number given to your volume.
5- Type "assign letter h" and press enter. Remember here "h" can be changed according to your wish.
Note- You should open cmd by clicking on "Run as administrator".
Congratulations your drive letter has been changed.