It is not cheap to do it yourself. You need a special computer, hard drive, and cables with techstream to do the update.
It is more cost effective to pay the dealer $200 to upgrade it for you.
Sadly Lexus locked down these years with a shitty way to update.
If you really want to find a way to hack your to update you will have to search russian forms as they have solutions but they involve removing the hard drive and trying to clone their files using a computer.
Sadly, I bought a 2017 tacoma with nav and the headunit/nav has nothing extra versus the lexus nav I had in my 2011 CT200h. Literally nothing new that is useful in anyway was added in 6 model years. No screen sharing capability.
just bluetooth and XM radio with the same NAV interface.
Toyota is a decade behind in NAV setups.
Had the dealer update my 2011 with a new disk a while back. Waste of time as even the new map is at least a year out of date. Tunnels had been built as well as new roads. Most are still missing!. I find the whole system to clunky and often it is easier to use google maps.