Select checks for updates when you close and reopen the app, and periodically as you use it. If there is a new version it will automatically download it in the background and let you know when it's ready to go.
You can also manually check by clicking 'Narrative Select Beta' in the main menu, then 'Check For Updates'. If there is an update it will download will install, then you'll just need to restart for the update to apply.
If you need to download and install Select again, click here: https://updates.narrative.so/select-latest.html