Registered domains cannot be changed once they're registered.
Note that your visitors can make the same typo you made, so leaving the typo registered and mapped to your blog will make it easier to find. This is often done with long and complicated domain names.
We offer refunds on domain names for two days after registration. However, note that when you cancel a domain, there is no guarantee that you'll be able to register it again for the same price, or that you'll be able to register it again at all. Since domains can be very hard to come by and considering that existing domains are relatively cheap to maintain, I would suggest you hold on to the domain you already registered, and add the other domain on top of it.
You can map as many domains as you want to the same blog - they will al redirect to the primary domain you choose.
To add the new domain you want, just enter it under Settings > Domains and click on Add Domain. Let me know if you also want to cancel the previous domain.
Let me know if you still want to cancel the domain. For help with adding another domain to your blog, see here: