Hello, and sorry you're having problems. The USB to PC cable for the Nuvi is available for purchase at the link below for purchase for $10:
Additionally, the update page is below and you will click on the Garmin Express link at that page to setup a Garmin Express account and then follow the instructions on the page to update your Nuvi 255W:
That USB Cable can also be purchased at a Best Buy shop if you'd rather not wait to order one.
If you need further assistance let me know. Before you go, PLEASE DO click at least a 3 STAR RATING, at the top of your screen (scroll up the page to get to the STARS). That is how I am credited for my efforts by the site. You can follow up any time as needed at no additional cost.
Do you need further assistance with this?