Open a command prompt terminal and navigate to install-root/bin/wadm directory and type the following command to reset the password:
./wadm reset-admin-password.
You will be prompted to type the new admin password. Type the password again to confirm. Now, you can use this 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.