How to extend main partition on C: drive?

you can not shrink the second partition from the beginning, only from the end, and you can not extend the first partition unless there is space directly at it's end. you would need to delete the second partition and then expand the first. This will lose all your files on the second so you need to back them up. And under Windows partitioning has NO advantage and a lot of disadvantages. Including seriously slowing disk access. Better to keep one large partition.