There's unfortunately no one-click method for this. As such, you need to do as follows:
- Sign up for a Screenly Pro account
- If you don't have your assets available locally, copy the folder ~/screenly_assets from your Raspberry Pi.
- Upload all assets to Screenly Pro
- Download the latest Screenly Pro disk image (available from within the web interface under Screen -> Add Screen).
- Flash out the new disk image to to your SD Card
- Pair your Raspberry Pi with your Screenly account