If we assume that it's a perfect isometric projection, here is one out of several ways to revert to a top view:

* Select the layer with the image.
* Press <kbd>Ctrl / Cmd</kbd> + <kbd>T</kbd> to enter *Free Transform*.
* Make sure the *Maintain aspect ratio* chain is unticked.
* Enter *173.634%* in the *Set vertical scale* field.
* Enter *45* in the *Rotate* field.
* Press <kbd>Enter</kbd> to *Commit transform*.

This gives you the following result:

[![][1]][1]

The angles aren't exactly 90 degrees. This must be because the image isn't a perfect isometric projection to begin with.

Scaling to 176% gives a slightly better result:

[![][2]][2]

You might be able to nudge your way to an even better result.

As you see, this only works for flat surfaces so the mountains aren't quite right. Nothing to do about that.


  [1]: https://i.sstatic.net/ia7T7.png
  [2]: https://i.sstatic.net/mjMbJ.png