How do I change a route on google maps on my iPod Touch?

I just got a fourth gen. iPod touch. I want to use google maps on a trip that I'm going on, but the automatic directions that it gives me are a little off. On a computer, I could just drag the blue line and it would automatically change it for me. I cannot seem to figure out how to do that on my iPod, or if it's even possible. Help?How do I change a route on google maps on my iPod Touch?
You need WiFi to access that aand you cant do it while on a trip

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