I am not sure if this will help anyone using a mac but it was very quick and easy for me when I did the following:
In your web browser input https://www.yourdomainname.com/cpanel
This will take you to your hosting account where you host your blog (you will need the username and password from them)
In the cpanel/back office of your blog scroll down the page and click the icon which installed your blog initially (my host is HostGator.com and my icon is Fantastico DeLux)
It is here where the option to upgrade to 2.7 was 30 seconds maximum and no glitches whatsoever.
I sure do hope this helped.
