Hi there,
There isn't a way to change the flow that clients follow on the Client Scheduling Page - they would need to select an appointment type first, and then their preferred date and time, according to the set availability.
As a note, if all of your appointment types are group class appointment types, you have no packages or subscriptions listed on the Client Scheduling Page, and you've selected the 'Daily' template on the Appearance page, your Client Scheduling Page template will default to the Class List template.
The Class List template lists upcoming classes by day, and clients can click the 'Sign Up' button beside the class they'd like to book into.
You can read more about the available templates in Scheduling below: https://support.squarespace.com/hc/en-us/articles/4402555019533