In the User Manager, set a new password for the admin account, and press Update Users.
I should hardly need to add, use something which is not guessable. At least six characters, preferably more.
You should see this confirmation that the password has been updated and a new salt created. The salt ensures that no two Mara sites with the same user accounts will have the same encrypted password copies. Which is important where users have -contrary to good security practices- set the same password on more than one site. You can make a note of the salt if you like, but it's in siteini.php anyway. This salt is termed a 'site secret' in some other CMS.
Since the change of salt has made your existing login invalid, you will be directed to log in again. You should do this before creating any more users.