I know there a thread and posts about this but for those who confused, this is a proper update for your Navigation on 2014-2015 IS250/350 from Lexus USA website. I've tested on my own car and it really works and improved the Nav system. I'm a Canadian Lexus IS vehicle they worked for both USA or Canada.

The issue on my car was The Nav system keep on rebooting and lagging while using the mouse, when the Nav reboot my Dashcam reboot along too i don't know why, did a research and found this helpful update from Lexus.com. i updated on Monday and the Nav system has improved and no more Reset/rebooting since, but will see how long it lasts, every i pray ! here the website below

https://securedp.lexus.com/download-app/getUpdates

13LHENNA-AB01_1525kwi << This is 2014 IS350 with Nav system
13LHENNA-AB01_1524kwi << This is 2015 IS350 with Nav system

Hope this helps ! enjoy your steady Nav while driving.
PS: before you start download, have yours flashdrive or Sdcard plugged in your PC ready at least 2GB free of space, once you done go to your car plug it in your USB port in your console box start your car go to Nav setting and click update, have a Cigarette or drink a can of Beer and wait until its all done.