The best way to do this is:

1) Back up DefaultAuthenticatorInit.ldift in your Domain Security directory and delete it
2) Run SetDomainEnv Script to set the PATH and CLASSPATH for WebLogic
3) Run java weblogic.security.utils.AdminAccount <username> <yournewpassword> .

NOTE: This command needs to be ended with  the DOT . and needs to be run under the security directory itself

This creates the New DefaultAuthenticatorInit.ldift file

4) Delete the DefaultAuthenticatormyrealmInit.Initialized file in the ldap directory of your domain(after taking the back up) and Restart the Admin Server.

Another Option: By deleting the LDAP files Directory (Taking the chances of loosing existing users information)

I have often seen people trying to delete the LDAP files directory( Speaking of which, I have done this too in testing environments…when I was a novice and took the pain of importing the users back in to the environment)

You could delete Embedded Ldap files directory and create the new Authentication file (DefaultAuthenticatorInit.ldift) and Re-start the Server, which would do the trick, but all the existing users’ information will be lost.  If you are fine with loosing the users or if you already have the users back up and would like to take the pain of importing the users….you could go for this approach.

1) Create the new Authentication file DefaultAuthenticatorInit.ldift with the new password as described above

2) Delete the LDAP files directory under the Domain Server Data directory and Restart the Admin Server

NOTE: This works for WebLogic 9 and Later versions…For earlier versions you would have to follow a different procedure.