How to Tell Flight Following You Want to Change Your Destination?
Just tell them.  Like you told us in the question.  Seriously, plain old English works just fine on the radio.  

Don't get all chatty, and be professional, but don't overthink things and feel you need to use a bunch of radiotelephony code words to get a simple point across. 

The most important thing is to clearly communicate intentions.  Don't tell them you are going one place and then "change" it when you actually planned on going there all along.  That's phony and unnecessary, and could confuse things if there any sort of emergency where knowing your planned route of flight or destination would be useful.

Simply tell them you plan to fly VFR up the coast to VanNuys, then turn Eastward and proceed to destination XYZ.  Or, more briefly:  "destination XYZ, via Van Nuys".  It's just that simple.

And consider filing a flight plan.