1. Hit CTRL-ALT-DELETE
2. This will bring up the following menu
Select "Change a Password"
3. You will be brought to the following menu
Type in your old password and then your new password twice, once to change it and once to confirm it. NOTE: Your new password must consist of a Capital letter, a number, and must be at least six characters long. Also, it probably shouldn't just be "password1234" or something to that effect
4. Once you have typed everything in, click the arrow next to "confirm password". And that's it! You're new password will now be in effect.
NOTE: The pictures above are taken from Windows 7, While it may look different, the steps in Windows 8 are the same