Back up important data
Open Disk Management
Check for unallocated space next to C drive
If unallocated space is adjacent to C drive, right-click C drive
Select Extend Volume
Follow the wizard and finish
If unallocated space is not adjacent, shrink or move the neighboring partition
Use a partition manager to move partitions if needed
Create unallocated space directly after C drive
Right-click C drive again
Select Extend Volume
If Extend Volume is greyed out, ensure the disk is a basic disk and not limited by partition layout
If needed, delete or move the partition between C drive and unallocated space
Restart the computer after resizing if prompted