Hi @apple, welcome to the community!
The ability to update the RX via the app starts from firmware 1.6. I see that currently you have firmware 1.5, this way you will need to update it via Flashtool. However, once you update the RX to 1.6 or a newer version, you will have the ability to update it via the app.
Hope it clears things up, if you have any questions, feel free to create a thread or contact us at support@emlid.com