It can be confusing. Just as an fyi, as you look through TomTom's support pages, the CX-5 has the NB1 unit.
First, you will need to install TomTom Home software to do the update.
See here for instructions based on your computer's operating system.
After you've done that, put your SD card in your machine and it should start up TTH. Before you do anything, I would open up File Explorer and cut/paste the entire contents of your SD card in a folder on your pc so if something goes wrong you know you have a working copy. I would also do this from time to time, like after major map updates.
Your first map update is free, using the Latest Map Guarantee. But there are other updates - map corrections, etc - that you can add for free.
See here for instructions on how to get your LMG. If it tells you that you're past the 90 day limit or something, you may need to contact TT support to get the LMG.
After that, whenever you put your SD card in your machine, it should start up TTH and it will check for updates.
See here for info on checking which version and map you have in case you're wondering.
Also, just as an fyi, when you turn off your Mazda, give it 20 sec or so before you take out your SD card, just to make sure it's not engaged. Also, if things ever get screwy and you don't know what's going on with your map, you can remove your SD card, start the car without it, turn off the car and reinsert the SD card as a way of resetting it. Sort of like rebooting your computer.
Hope this helps.