While the Google Experience launcher comes with Android 4.4 on the Nexus 5, for now it is an exclusive for the latest stock Android handset. But there is a way to painlessly remove it from your Nexus 5. Just keep in mind that the Google Now integration with the homescreen will disappear when the old, familiar launcher replaces the one that comes out of the box with the Nexus 5. Same for the translucent status bar.
To remove the Google launcher from the Nexus 5, go to Settings> Security. Click on the unknown sources box. Click here to load the Nexus 4 launcher on your phone or computer.
If you downloaded the file on your PC, the next step is to use the microUSB charging cable and hook up your Nexus 5 to the computer. From the PC, double click on the Nexus 5 icon and you should be at internal storage. Drag the file you downloaded into the root of the storage. Exit and disconnect the Nexus 5 from your PC. Using whichever file manager you want, find the file on the phone, tap to install, and you're done.
If you downloaded the Nexus 4 launcher on your Nexus 5, go to the app drawer and go to the Downloads app. The file should be there. Click on it to install and you're done. Don't select the launcher as the default so this way you can go back to using the Google Experience launcher once you start missing it.