Lumia Cyan update is a parellel upadate to Windows 8.1. It is available for Lumia 620 and is a nice upgrade to be honest. It will allow you to install apps on the SD card, brings a voice enable personal assistant, enables folder creation to group related apps, Storage Sense just like the Wifi Sense and more.
It will take around 2 hours to install on a connection with download speed of 200-300 kbps. It is suggested to have 1.5 GB of free space to install this update. Though it is difficult to tell the exact size but calculating based on the time it took me to update, it should be around 400-500 MB