Regs: if you're impatient, and can't handle the cable trick, you can always try this at your own risk: http://windowsphonehacker.com/articles/want_windows_phone_7.8_now_try_this_easy_tool-01-31-13
according to the website:This tool will install the official, RTM Windows Phone 7.8 update today, right now. It's easy. Just click Install 7.8, then select the desired language packages. Then wait.
this is, by far, the easiest method :-D (i have done this on a couple of phones - a lumia 800 and a lumia 900)