A 1 GB memory card is large enough as sonar2000 stated.
Try searching for the install instructions.
I would:
1.) Export all of the Nav Data from the unit and save it on your computer before starting the update.
2.) Delete all of the Nav Data from the unit and use the Format Nav Directories (if your unit has new enough software, otherwise just Delete the Nav Data).
3.) Reset the menus back to the factory defaults (Restore Defaults menu).
4.) Fully format the memory card I will be using.
5.) Download the update file directly to the memory card.
6.) Perform the software update with the unit on your boat as this should provide a constant steady power supply for the unit.
7.) Follow all on-screen instructions and answer all on-screen questions (first one “Yes”, second one “No”).
8.) Allow the unit to complete the update, power off and back on again before removing it from the power source.