Please be aware, throughout the OS update, your Launcher may reset, causing some of your apps to be moved throughout the homescreen. If you notice any apps missing, please be sure to scroll across the homescreens, to find your app, and reset it to your preferred location!
Keep in mind, Marshmallow offers some great new SD card Storage features outlined here: https://source.android.com/devices/storage/adoptable.html. Be sure to check it out to learn about the new way your SD card can really help improve your storage.
Because of the new Storage feature, it is important to note if your apps were saved to the SD card, you will be unable to access them once your upgrade is complete, be sure to back them up to you can restore from your google account.
Make sure to back up all your apps and data, as an OS upgrade could wipe all this stuff from your device!
In order to unzip and install the update, your device will REQUIRE a SD card with at least 4GB of space available.
Navigate to the Settings Menu
Select About Device
Select Wireless Update and then Check for Updates.
Hit the Update now button, and your installation will begin.
Upgrading can take up to 30 minutes while your device installs the update.