When map updates are available, you'll get a notification asking you to refresh your maps. Just tap the notification, and the update will happen automatically.
During the update, HERE WeGo will download and install new maps, and then it will delete your old maps. If there’s not enough memory available on your device to handle this, you could delete the old maps yourself first, then tap the update notification, which will download and install the updated maps.
The update will run in the background, so you’ll not be interrupted by it. To use your new maps, close HERE WeGo completely (make sure it’s not running in the background) and then restart it.