Turn auto-sync on or off for your Nexus phone

On devices running Android 5.0 and up
On your mobile device, open Settings .
Under "Personal," touch Accounts.
In the top-right corner, touch menu Menu.
Check or uncheck Auto-sync data.

On devices running Android 4.4. and below
On your mobile device, open Settings .
Under "Wireless & networks," touch Data usage.
In the top-right corner, touch menu Menu.
Check or uncheck Auto-sync data. 

資料來源:
https://support.google.com/nexus/answer/2840875?hl=en