you'll need to get into the bios and set it there
IF you dont know what your doing dont do it as altering anything in the bios APART from the clock can make the computer stop working.
If you need to know how to get into the bios have a look here
http://www.fixmypcuk.com/index.php?page=Bios
Follow the instructions on how to get into the bios then ignore.
The clock selection is on the 1st screen and it tells you at the bottom which keys to press to change it.
once your finished just exit saving changes.