How do I replace a leaky secondary water valve?
Without seeing the actual valve type and piping arrangement it will be very difficult for anyone to tell you what you are in for. 

One thing I know for sure: step #1 is digging it up! Once it is exposed you (and our on-line community) will have a much better idea of needed repair work, if you add pictures to your post.

Questions:
1) Is it a continual leak that keeps the area flooded like in the picture? If so, you will need to effect a positive shut off upstream and let the water soak in or dry up before excavating. You will likely need to do this anyway to proceed with the repair or replacement.

Judging by the type of handle I see in your picture, I suspect it might be a valve like this:


If it is a brass valve like this, they are usually fairly simple to repair. replacement is not always as easy.