Basically it's pretty simple and very good explained in the documentation at yubico.com. You'd first of all want to download the Yubikey Configuration Utility, which can be done at
http://www.yubico.com/personalization-tool.
When you start the software, there are different modes that you can use. In your case you'd need
Static Password. You should then choose
Scan code ... there you go, you can set a password by entering it in the corresponding area and write it to the stick by clicking on
Write Configuration.
NOTICE: Make sure you activate
Configuration Slot 2 and not overwrite your factory settings in slot 1.
Best regards,
GByte