Disclaimer: I am not to be held responsible for your mistakes or misplacement during this simple procedure. Opening up Logitech’s product DOES void your warranty, so proceed with caution. Logitech have a really good support base and they will assist you and mostly like offer a replacement as this is a well-known issue.
Link to PDF Version: http://dl.dropbox.com/u/5658586/518/Fixing_Logitech_MX518.pdf
Link to PDF Version: http://dl.dropbox.com/u/5658586/518/Fixing_Logitech_MX518.pdf