Lumia 520 being released recently there will not be any updates for the phone as of now. But in future when you want to check for updates follow the below steps,
From the home screen, swipe towards the left where you get all the icons. Drill down and select Settings. From the system application setting select phone update. When you check for phone updates make sure you have your Wi-Fi or 3G on.
Tap the ‘check for updates’ button, this will check for the updates and will show you if any update is available. If not it shows the message ‘your phone is up to date’ as shown in the screenshot below.