The answer depends upon how the heads are connected to the buried lateral pipes.
If funny pipe is being used, you replace the funny pipe with longer pieces of funny pipe.
If swing joints are being used, you'll have to make one of the "arms" of the swing joints longer.
If simple risers were used, you'd have to change over to something like funny pipe.
I would start by digging up around one of the heads in the middle of the run. Keep digging until you either reach a "T" connector or something else that seems to indicate you've hit the termination point for that head from the lateral line.
Then from there you should be able to either figure it out by yourself, or you can come back here, tell us what you found, and more detailed answers can be provided.
