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There are two types of updates that can be performed, software update and map update. The software updates are free, and the map updates have a cost associated with them. Map updates are typically $49.99 for a one time update, or $89.99 for lifetime map updates (though cheaper upgrade prices can be found from online retailers such as Amazon.com.
With either update, you will need a USB cable to connect your Garmin to the computer and software titled Garmin Express to install your updates. You can download the software by clicking here.
The map updates will need to be purchased through the Garmin website if you wish to download them to your device, however you do have the option to purchase them on a microsd/SD card. I would recommend purchasing the updates on the SD card as chances are today's maps are far larger in size than your Nuvi 255W can handle.
I hope I have answered your question. I will be here to answer any follow up questions you may have.