Step 1: Login to admin area
Step 2: Select Users and then User Manager.
Step 3: Select the check box next to the administrator user name and click on the Edit button located at the top-right Joomla menu.
Step 4: In the Edit Profile section you should enter and confirm the new password.
Step 5: Save the change.
Next time when you want to open the Joomla’s administrator area use the new admin password.