Haha, rude (the grammatical error) :<
Your site is running an old version (2.x)
SS version can be checked in a number of ways.
In the CMS, hover over the logo in the top left.
In older versions (if the menu goes horizontally accross rather than down the left) then the version is (usually) printed in text in the lower right of the CMS.
The origin of this number found in one of the following:
if the CMS looks similar to the one at http://demo.silverstripe.org/admin
- then you've got 3.x
If the above mentioned file contains just $URL, then you've got 2.4.6+
if you've got sapphire instead of framework, it's a reasonably safe bet you've got 2.x.
Upgrading is a simple process of (backing up first!) replacing framework & cms with the updated version... NORMALLY (not in your case, unfortunately).
There are siginifcant API changes between 2.x and 3.x, as one would expect. There are minor API changes between 3.0 and 3.1 that you will need to edit.
Details can be found in the changelogs on http://docs.silverstripe.org on the major and minor version jumps.
You won't need to upgrade to fix that error though, luckily.
You'll find the translation files sitting in sapphire/lang or cms/lang, and the offending text in whatever is set in mysite/_config.php as the default locale (.php)
Although more likely they'll be sitting in mysite/lang - because looking on github, the translation files have the correct "You\'re" all the way back to 2.2 - so it could be a custom translation someone has added (eg. en_AU)
It it's not the admin area (URLs: site/Security/* or site/admin/*) then you're probably viewing a custom 'front end back end' that a developer (who has trouble with grammatically correct English) has put together.
In which case you should be looking in mysite/templates or themes/<themename>/templates (where <themename> is the value you'll find set in mysite/_config.php)
In short - just ssh into the server and use grep -rin 'your logged in as' installdir/*