Open a command prompt terminal and navigate to install-root/bin/wadm directory and type the following command to reset the password:
You will be prompted to type the new admin password. Type the password again to confirm. Restart the server in order for the password to take effect. Then use the new password to login to the server.
While changing the password using set-admin-prop command you must restart the Administration Server from install-root/admin-server/bin/restart directory. Do not use restart-admin command to restart the server.