If you haven't received the initial WP7.8 update on your device, officially or otherwise, then the update will not affect you. To put it simply, you'll be stuck with the current version of Windows Phone 7 you have already installed on your phone.
Should you be interested in updating, here's a tool that simplifies the process:
SevenEighter: Want Windows Phone 7.8 now? Try this easy tool (Update: Now supports 8862!) - Windows Phone Hacker
*Only applies to the Nokia Lumia WP7 line of phones*: if you update via the tool provided within the link, be aware that the process only updates your phone's OS Version, not the firmware that comes with the update that you receive via official means. To download the firmware that coincides with the WP7.8 update, go here:
[CAB] Official WP7 Updates List up to WP 7.8 + ALL LANGUAGES complete + OEM updates - xda-developers