set user-name

SET USER-NAME name   -  define your user name for MM

The USER-NAME variable defines your user name.  For the general user
  population, it cannot be changed, and it is equal to your user id (login
  name).  If you would like to change what is in the FROM field of your 
  outgoing messages, SET the PERSONAL-NAME or DEFAULT-FROM variables.

You can only change USER-NAME to another one with the same uid.

For more help, type "help" and one of the following:
(at any prompt:)  save-init   set   show
                  set personal-name   set default-from