The Nest Thermostat is pretty easy to use, but it isn't so apparent how to turn off the thermostat when you don't need heating or air conditioning on.
Sure, you could just set the Nest to "Away" to stop it from heating and cooling, but that won't work very well if you have Home/Away Assist enabled. Luckily, there's a really easy way to simply turn off the Nest Thermostat. It's not directly out in the open in the Nest app, but it's easy to get to, and you can also do it from the Nest unit itself.
From the Nest App
Open up the Nest app on your phone and select your Nest Thermostat on the main screen.
Down in the bottom-left corner, tap on "Heat". It also might say "Cool", depending on the weather in your region at the time.
When the pop-up menu appears, select "Off". This will turn off the Nest Thermostat until you turn it on manually again.
On the Nest Thermostat
Click on your Nest Thermostat unit to bring up the main menu.
Using the silver scroll wheel, navigate to "Thermostat" and select it.
Use the wheel to select "Off" and push on the unit. Your thermostat will now be turned off.
How to Adjust the Nest's Safety Temperatures
Keep in mind that even though it's off, the Nest Thermostat still has safety temperatures that it will act upon. So if the temperature in your house reaches a certain level (the default is 40 degrees Fahrenheit), the Nest Thermostat will automatically turn on and begin heating the house to prevent pipes from freezing. This is great if you ever forget to turn the thermostat on while you're away from home for a significant period of time.
To adjust these safety temperatures, you can do so from within the Nest app. From the main screen, select your Nest Thermostat.
Tap on the settings gear icon in the top-right corner of the screen.
Select "Equipment" from the list.
Tap on "Safety Temperatures".
From here, you can adjust the safety temperatures a bit.
By default, your house can get down to 40 degrees Fahrenheit, and there's no default maximum temperature that will trigger the AC to kick on. However, by holding and dragging on the dots, you can set these to specific temperatures, but you can only go as high as 45 degrees for the heating and as low as 95 degrees for AC. You can also turn off both if you want, but it's always a good idea to set these so that you don't cause any trouble in the future.