The Daylite app is updated on the iOS and iPadOS devices via the App Store.
If an update for Daylite is available, it will be shown as notification within the App Store. Depending on the settings on your iPhone, this happens automatically or has to be done manually.
Relates articles
How do I save an email in the Daylite app for iPhone/iPad?
How do I add an appointment in the Daylite app for iPhone/iPad?
How do I add a contact in the Daylite app for iPhone/iPad?
How can I customize the menu bar in Daylite Mail for iPhone and iPad?