It looks like you can just dunk the bottom part where the dirt is. Pull the lining out so it doesn't soak through. I've never had a problem with water stains from only washing part of a bag. If the stains don't come out, try Soilove. I've used it on a glam and it works great.
Thanks whateve and everyone else and thanks to catshandbags for posting pics. I'm looking at one on the 'bay with stains in the same area! If I can get it cheap enough it should be worth it!