If you want to set the admin password using curl then you can use this command to do it.
curl -u admin:<OLD PASSWORD> -Fplain=<NEW PASSWORD> -Fverify=<NEW PASSWORD> -Fold=<OLD PASSWORD> -FPath=/home/users/a/admin http://HOST:PORT/crx/explorer/ui/setpassword.jsp
If you want to set it from interface then you can do it by going to
http://localhost:4503/libs/granite/security/content/userEditor.html. In search box search for Admin. You will change password after you select the user. Change the password as required.