Replace a Dead App Store app

I have a 13-inch MacBook Pro (Mac 7,1). Processor: 2.4 GHz Intel Core 2 Duo Memory: 4 GB. OS X is Snow Leopard (10.6.8) Browser: Firefox 36.0.4

A year ago I noticed that My App Store app is damaged. In fact the app contains no data -- it has zero KB. This dead App Store app prevents me from downloading any App or software from the App Store. I have been trying to upgrade my current OS X 10.6.8 to Yosemite which is a free upgrade for me. But I am not able to do so because the moment I click on the button "Upgrade" to Yosemite from the Apple site, it tries to take me to App Store and immediately I get a message from Firefox to the effect that it cannot find the address because there is no program available...

So my question is how do I install a new App Store app? I can easily trash the dead one. The only answer I have received so far from someone is that i have to reinstall my old original version of OS X and upgrade all of my original apps such iTunes and reinstall all the other apps (I have many) I have added from various sites. This would be a risky and very time consuming job and I want to avoid it.

So please can anyone help me install a new App Store app without reinstalling my very old original version of OS X?

I would very much appreciate any assistance.

Thank you.