It's common to meet issues like Low Disk Space warning on C, after you add more and more data to your drive. You can increase the capacity of partition C to solve the problem. However, you will find the option Extend Volume greyed out in Disk Management when there is no contiguous unallocated space behind C drive. You can get qualified unallocated space by shrinking partition and you need to delete the partition behind C drive to gain the unallocated space that can be added into C drive.
To allocate free space from one partition to another partition such as partition C without deleting partition, you can use Qiling Disk Master's Allocate Free Space function. It allows you: