Shrink Disk Space of Virtual Machine in Esx5.1

Hi,

I am using ESX 5.1 and my datastore size is 100 GB. I have created one virtual machine and its C drive space is 50 GB. Today I checked that usable space for C drive is only 6 GB. Now I want to reduce the space of  C drive to 15 GB. What are the possible ways ?

Thanks.