You can access the wheel by unzipping the inner nylon.
"We unzipped, my husband used three different tools and replaced one and then the other. He greased the fitting too. It wasn’t difficult once he knew what he was doing, he used the on line guide. For twenty dollars, it was worth it because after multiple overseas trips and the cyclone Gabrielle silt, it’s still going strong! A little bag tip, use candle wax on all your zips. They run more smoothly."