Find "Email"
Press the menu icon.
Press Email.
Select email account
Press the Menu key.
Press Accounts.
Press the required email account.
Select default email account
Press the Menu key.
Press More.
Press Account settings.
Press Default account to select the account as your default email account.
When the box next to the menu item is ticked (V), the account has been set as your default email account.
Exit
Press the Home key to return to standby mode.