As of 25th October, Google (which owns Nest) has shut down the Nest platform. This means that you can no longer use your mobile phone to adjust the temperature of your home remotely. An open source project has been created to replace the firmware (basically the operating system) of 1st and 2nd generation thermostats so they can be used without Google’s platform.
Enter Nolongerevil.com
The open source project is aiming to provide an update for the firmware using a system known as DFU mode (Device Firmware Upload)
There is a guide on the website nolongerevil.com
Bear in mind this is experimental software right now, and you risk damaging your thermostat.