How can I change the locale setting?

["Thomas H. George" <xyz@spininternet.com>]

I must have missed something very elementary.  I can't find out how to 
change the locale setting from POSIX to en_US.

locale -a shows both are available.  apropos locale provides a long list 
of locale programs including setlocale.  The man page for setlocale 
specifies the header to use in a C program.  I am not able to whip up a 
C program at the drop of a hat though I can work one out if forced.   
Still there must be a trivially simple way to change the locale setting 
though I can't find one in my reference books.

I would appreciate it if someone could take a moment to point me in the 
right direction.

Tom George