How to expand partition into free space that lies before it
You can't do what you are asking, per se.  What you would have to do is create new partition "above" your existing partition, copy the data there, kill the partition below, then expand the partition with the data into the newly created free space.

You can do this with  Disk Utility.  OS X Daily has a really good write up on it (limited on how to resize the partition)

However, I am personally not a fan of resizing disks with any sort of data on it.

I prefer to follow these steps:


Copy all data from the drive to my desktop, a USB, burn a DVD;
basically make a backup copy of the drive.
Using Disk Utility, erase and repartition the drive.
Copy data back to the drive.


Yes, it's a bit old school, but it is super reliable (not that the partition resizing is a failure by any means).  I am just old school and this procedure gives me peace of mind.