I applied to the University of Windsor's MBA program in January. On my letters and resume that I submitted I said that I was expected to graduate by the end of the Winter semester this year. It turns out I have one class left that I have to take in summer session which can be either 6 weeks or 3 months (until June). I also wrote on my application that I would be taking my GMAT come the summer as well. How will having to stay back in the summer to take one extra class affect my application as now the information about graduation will be wrong?

You will need to ask them. The problem might be getting a visa without a completed degree.

Yes, just let them know the situation and see what they recommend. In terms of admissions, there might be some flexibility, but if that's the case then you'd need to show that you're motivated and demonstrate your continued interest. Why do you need to wait until the summer to take the GMAT? Typically you would send your GMAT score when you apply. Not doing so might not look so good.