Where it says Free Space. The rightmost box with the Green. If you right click in that box there should be a choice to Delete it. Then it should say Unallocated. Once it's unallocated, you should be then able to expand C:.
Especially if you have done this type of thing often it's a good idea to get an image backup program such as Macrium Reflect or similar and save all your partitions to an external USB drive if you have one.
This case should be simple though. If you unallocate and it still doesn't give an option to expand C: then something is weird. If anything is weird just abandon all changes.