John,
You will need to put the SD card into a PC with a SD card reader.  Then go to www.naviextras.com , create a login and download the "ToolBox" onto your PC.  From there, you should be able to open it up and it will auto-recognize the SD card as a 700 or 720.  It will then tell you of any updates that are available.  It's not very difficult, however it is not a simple as updating the Aviation Data.  Thankfully, its only update every 6 months to a year, depending on the number of changes. 
They have changed this site a few times so there may be a few minor changes.  Once you get the ToolBox on your PC, it's pretty simple from there.  The ToolBox occasionally updates as well, but that is prompted once you open it.   I hope this helps.