It is kind of hard to fix the problem with mesh already attached
You could remove it and reattach though,after filing the edges very carefully and then reattaching from the outside.I know a lot of people would say just use hot glue but that is not a good idea since a hamster would just end up chewing the glue off and eventually bring back the initial problem(so it's just a bandaid,in a way,rather than a permanent solution).