Tap the stock Email app open.
Tap or long press the Recent apps key to open up the additional options and then select Settings.
Tap Manage accounts.
Select the account you want to set as default and then enable it by checking off the box next to it.
I don't have a Priv so I'm not sure if this is possible directly from the Hub. However the above instructions (or a close variation of) should work as this is the instructions for Samsung S5.