If you fished on a reef, click on the map icon to the right of the Reef field. If you didn't fish on a reef, the next best option is to click on the map icon to the right of the Location field. Either of these options will display a map allowing you to select the location where most fishing activity occurred.
With the map option, click on the black box to select a reef or click another location within the grid. With your location selected, click the Done in the bottom right corner.
To learn more about how to navigating the map, please read our article on Map Navigation.