A little trick when you hit a locked installer in Akeeba
If you ever need to restore a backup with Akeeba Kickstart, follow their documentation video from their website and before you click ‘run the installer’, read this:
If you have ever tried to run the installer and it says “The installer is locked”, there is something else you need to do first. After extracting the backup archive with Kickstart and before you click on the “Run the Installer” button connect to your site with FTP or SFTP. Delete the file installation/password.php. Now the installer will be unlocked.
You can use a programme such as Filezilla to ftp onto the site. Then, look in the installation folder for the password.php file.