It depends on where the unallocated space is to the right of the C: drive in Disk Management. You may be able to use Disk Management to extend the C: drive into the unallocated space. Partition or Volume - Extend
If it will not let you do it in Disk Management, then you may need to use a 3rd party program like the free Partition Wizard
to extend C: into the unallocated space instead.
Hope this helps,