Try Tools/Accounts - Mail tab, highlight the account you wish to be the
default and click the Default button.  If that's not what you are referring
to, please post back with more details as to exactly what you need.

You can also take the old account you no longer wish to use and either
delete it, or double click it in the list, and remove the check to "include
this account when receiving mail."

Well, I THOUGHT I was being explicit, but I'll try harder.

ISP info: (AT&T)
There are two email addresses set up for the same man.  One is JSmith, as an
associate member of HJones.  The second is J.Smith (note the dot), as an
independent member on his own.  He had to create his own because HJones is
getting DSL, which will severely restrict JSmith's abilities.  Both JSmith
and J.Smith wii be active for some time.

OE info:
OE is dealing with a single "account", namely AT&T's
(postoffice.worldnet.att.net).  In the profile associated with that account,
we showJ.Smith@att.net for the email address;  nevertheless, when we load OE
and download mail to the Inbox, we continue to get mail only for JSmith, and
not for J.Smith.  So there has to be something we're missing in OE that tells
AT&T whose mail we want.

I hope this will help you to help me.