Nokia T21 user guide

Tap Settings > System > System update > Check for update to check if updates are available.

When your tablet notifies you that an update is available, just follow the instructions shown on your tablet. If your tablet is low on memory, you may need to move your photos and other stuff to the memory card.

Warning: If you install a software update, you cannot use the device until the installation is completed and the device is restarted.

Before starting the update, connect a charger or make sure the device battery has enough power, and connect to Wi-Fi, as the update packages may use up a lot of mobile data.