clean out all the sh!t that's under the lip of your arch. if you don't it'll show up on the outside. if you have plastic arches like me then remove these. then using a hammer and dolly gentley tap the lip up and out of the way. You don't have to do the whole arch, just the top 1/3 where the tyre would hit. do the tapping in small ammounts and you should be ok. the plastic arches will reattach at the bottoms fine, but the tops will need to be stuck back on as you've just rolled away the point where they use to rivot/screw on.
failing that you can make an arch rolling tool with a scissor jack.