1. Right-click My Computer and choose Manage -> Storage -> Disk Management.
2. Select the adjacent disk of the disk you want to extend, right click and choose Shrink Volume to create some free disk space.
3. Select the disk you want to extend, right click and choose Extend Volume to expand the partition.
While please note that there must be unallocated space behind the partition which you are going to expand if you want to expand partition with Windows built-in Disk Management utility. Otherwise, you might find that the Extend Volume option is greyed out and not available. At this moment, you have two choices to solve this problem: delete a partition or expand partition with third-party partition manager software.