Upgrade problem "Warning: No configuration file was found."

Trying to upgrade from 2.35 to 2.4.

Working locally on a Mac/MAMP.

Downloaded the 2.4zip file unbundled and moved public html files into a duplicate of the 2.35 installation.

Then go to  install/upgrade.php in the browser and get a screen with "No installation detected" / "Warning: No configuration file was found."

In the install folder I can see a config file. Tried dumping the install folder and reloading a clean(new) copy but the same result.

Can anybody give me a steer as to what I should be looking for?

Dave

To upgrade, the upgrade script searches for an existing file includes/config.inc.php.
The install/config file you see is a template for creating includes/config.inc.php upon new installations.