I'm trying to update my mac but my mail won't respond to allow my mac to shut down. How do I fix this?

I'm trying to update my mac OS X Yosemite. I've been trying since I received the mac in December. My sibling and I received the same mac on Christmas. Hers was straight from the apple store. Mine was from Future shop. Hers came with the OS X Yosemite. Mine did not. I will have technical difficulties every month but she doesn't. At first it was a wifi connection issue. We have now resolved that and I"m trying to update it but my mac will not shut down. The update requires a restart in order to complete the process. I was previously having difficulty opening my mail. I forced my computer to shut down and that allowed me to open my mail and receive and sent mail. But now the message that appears on my computer is something along the lines of that the mail will not respond then you can choose to "retry" or "cancel" and when i hit "retry" it doesn't waste more than a second and I'm back to the original message. Please help me solve this problem.