If you are on a good internet connection you don't have to do anything. The weather data will download to SeeYou automatically in the background:
If you want to be in control of what gets downloaded when you can choose what SeeYou does when new weather data is available:
The option "Notify me before downloading data" will open a notification in the Windows taskbar. Clicking on the notification will open the Top Meteo dialog and then you can choose what to download then. You can download entire region or any specific day separately that way significantly reducing the download time and size.
Typical average Download sizes:
Whole Europe: 5 MB / day
Whole of Africa: 1 MB / day
Whole USA: 15 MB / day
Size depends on how good the weather is going to be - better weather means more data.