How to switch the view to have Diagram first ?

Dear David,

You can find that information in ARIS Help (ariscloud.com) under Manage fact sheets. In your modification set you edit an item, e.g. process, where you want to change the order of the available fact sheets. In your case Diagam in front of Overview:

That changes the order of the fact sheets immediately:

However, with ARIS 10.0 SR22 you can get your modification set automatically migrated to the new layout where the Overview includes a model preview which might suit your audience even better than just the pure Diagram view. Here is more about ARIS 10 SR22: Redesigned and adaptable overview fact sheets (content overviews) | ARIS BPM Community (ariscommunity.com)

Cheers
Runè