Depending on your desktop environment, you may just see a blanket media player option. Regardless of what you did to get there, once you’re there scroll down until you see Video and Audio player options. If you’re an LXDE user, then you’ll want to click on the LX Menu followed by Preferences and select Default Applications for LXSession. Xfce4 users won’t have to worry because that desktop environment primarily uses the above method for setting file associations. Older versions of Ubuntu stored this in System Settings under System Info, and there may be a similar applet found for MATE users and the like. Users of the Ubuntu Dash will want to select System Settings, click on Details and then click on Default Applications. You’ll need to open the Default Applications configuration applet. These instructions will change your default media player across your desktop environment for all file types, but this can easily be undone so you won’t do any lasting damage if you pick the wrong one.

If you don’t have a media file picked out, then you’ll need to do one extra step but you still shouldn’t have to play around very much to get the job done.

How to change default video player