You have to have a login on that site, and then select the menu items as follows
"Home"
"Payable Access"/"Map and Software Updates" (1st 3 years of owning car are free, I believe)
"Lexus Premium system (MM17 & MM19) Maps" - select the "Update" button
Then follow the instructions.
Yes, it takes about an hour to download the map updates to your pc (or Apple if you are so inclined....), then you put the files on a USB stick, properly formatted as per the instructions, and then put the USB stick in your car and perform the update. This step also takes about an hour and make sure the car is on (in the READY status" otherwise the car will switch off after 20 minutes.