Hi,

Make sure to switch to timeline mode, so you'll have full control over your arrangement. You will see that the video footage you brought in takes up two tracks - one for the video and one for the audio underneath the video track. If you want to mute that original audio altogether, you could just activate the mute ("M") button in fron the the audio track. You could also use the volume control handle in the center of the audio track and pull it down to reduce or completely mute the volume.

Also, you could ungroup the video and audio track by using the unlock button (or CTRL + M). After doing so, you could select the audio track separately and just delete it from the arrangement altogether (Delete key). This way, if you're not using it, you don't have to see it at all anymore. It's all just virtual editing, though. You're not deleting the audio from your actual source file, so don't worry.

After this is done, you could just bring in your desired background music as an audio file or from CD via the media pool and put it down on one of the tracks next to your video footage.

I hope this helps!