Settings → Connections, then press Make default on the account you want. The current default carries a default chip on its card. Or just ask Nell — "make my Outlook calendar the default" or "set my default email to my work address" changes it on the spot.
The same control exists for both types: the default calendar is where new events are created, and the default email account is where new mail is sent from. Naming an account in the moment always wins ("put it on my personal calendar", "send that from my work address"), and replies always go from the account that received the original message, whatever the default is.
The default option appears once you have more than one account of that type connected — with a single account there is nothing to choose.