To update your Garmin Nuvi GPS for free, download a map update within the first 90 days of using the device through the Garmin nüMaps Guarantee and install the update to your device, as of 2015. Another option is downloading a free open source map and installing it on your device.
The Garmin nüMaps Guarantee allows you to download one free map update within the first 90 days of your device connecting to a satellite. You can download the update using the Garmin Express application. Download the application from the website. After it's installed, connect your Garmin Nuvi to the computer with a USB cable and the application finds the correct map update.
If the 90-day period has passed, Garmin requires you to pay for map updates, unless your device has lifetime updates. If so, you can always download free map updates for that device.
A variety of free open source maps are available online. Since Garmin doesn't create these maps, there is a possibility that these maps might not work, may harm your device or invalidate your warranty.
Before purchasing a map update, consider how long you've had your device. If it is an old device, you may want to buy a new GPS device that has current maps, instead of spending money to keep an obsolete device updated.