You'd be amazed how many people don't know what a "Robot Car Kit V2.0" is. Can you provide a link to what it is that you're talking about?
Also please post the program that you are currently using so we know what you want help to modify. To post the code please use the Reply button then the </> code tags as described in the "How to use this forum - please read" sticky at the top of each forum.
There are so many possible ways to change the speed of motors that we really do need to know where we're starting from.
Steve