Here's what I've found:
Open your mail application, and make sure you see your mailbox list on the left. (If you don't, go to the mailbox menu and click View > Show Mailbox List).
Look at your inbox in your mailbox list on the left. There's a flippy triangle next to "Inbox". Click the flippy triangle so that you see each individual account listed under inbox.
Finally, from that list of individual accounts, drag the one you want to be your default account to the top of the inbox list.
That should do it!