If you right click the C:\ partition, can you not use the native "Extend Partition Wizard" to extend it into the free space?
I would expect it to indicate that, that space is "Unallocated"?
But then, I would not recommend a single partition of 500GB for the system drive. Unless you have a specific requirement for it I would allocate no more than 100GB to drive C and create a second partition for data storage.
And before you do anything ensure that all your data is backed up and you have the means to run a full recovery install in case you lose or corrupt your operating system.