Obtain a blank microSD/SD card (up to 1 TB, depending on your plotter). The chart file for a single region can be quite large, requiring significant space.
Download Garmin Express, plug in your plotter, and see what Garmin offers you. If the 2021 version appears in your library, great. If not, bite the bullet on the $149 update. Your keel is worth more than the cost of new charts. garmin bluechart g3 download 2021