1. Access the Settings menu of your Android device, clicking on the wheel present in the pop-up menu displayed dragging down the status bar at the top.

    2.Scorri the items in the menu and select Application Management.
    Of all the applications in your device, download and grouped in Now Playing, click on the one you want to disassociate.
   3. In the following form, application information, click the Delete button default in the section Start by default.
   4. At this point, when you click on a file type of music, present in your device, you'll see the pop-up form appear "Complete action using" in which you will see a list of all the applications you can use to open that file type.
   5. Select the new application you want to use and click Always, if you want to reconfigure the default setting to open that file type, or the Solo button this time to use your chosen application only in the current case.

Get it?

I'm trying to understand you, but I'm not seeing the menu options you list. I can go into my device settings menu, and select App settings. But there is no option listed in the settings menu for either Firefox or the Youtube app installed to delete Start by Default (that doesn't appear anywhere). Anyway, my problem is NOT with files downloaded to my device opening with the wrong app. My issue is with URL links I click on while browsing the internet on Firefox. Within Firefox, when I click on a link to a YouTube video, I want the YouTube app to launch and play the video. Currently Firefox loads the mobile version of the video, which is not what I want.

Sorry for my bad english...try again:

1. Goto to the settings.

2. Select in the list App.

3. Select All (here you see all the app installed on your tablet).

4. Select Firefox or youtube.

5. In the new tab search, at the end, the bottom 'Clear defaults'.

Now try to open a link on firefox and tell what appen..get it?

Thanks for clarifying. I completed the steps you provided and rebooted my tablet. Firefox still opens YouTube links in the browser, not in the app.

Those instructions are irrelevant. The problem here is that Firefox is preventing Android from capturing links clicks for apps that request them. The YouTube app is designed to capture "youtube.com" links, for example, but Firefox is bypassing the normal process and loading these links. You then have to click on the video on YouTube to load it in the app.

Please correct this behaviour, it's not the correct approach under Android.