This sounds like a corrupt download for the installer, which is possibly due to temporary Internet files causing the issue. First, delete the old download, if it appears in Applications, remove it by dragging it to the trash, and restart the computer, Then, clear the cache for your Internet as follows:
Here’s a quick introduction into how to delete browser cache on Mac.
Chrome clearing cache tutorial
Here’s how to clear browser cache in Chrome manually:
Firefox clearing cache tutorial
Here’s how to delete cache in Firefox manually:
Safari clearing cache tutorial
Safari is a little trickier than the rest of the browsers. You could remove caches together with all the other website history through History — Clear History in menu bar.
But if you need more precision, here’s how to empty cache on Safari browser:
Make sure you close/quit the browser and restart it after clearing cache.
Download the Installer again, and try it one more time, if it fails, it is time to consider CCleaner, which is available at http://www.piriform.com/ccleaner
Please let me know if you need anything further, otherwise please don't forget to accept and rate! :)