Since the unallocated space is at the front of the disk, you can't expand the currently existing partition to take up that space. You could move the currently existing partition to the front of the disk and then expand it to fill up the rest of the drive, but this is a fairly risky operation as it involves moving the physical location of all the data on the partition from one part of the disk to another.
You would certainly need to back up your data before doing something like this, and at that point, it would just be easier to temporarily copy the data to another drive, reformat the disk to suit your liking, and then copy the data back onto the freshly formatted partition.