Try downloading the video and take your computer and do it there. I'm sure there is a video editor you can download to accomplish what you want.
Hi, you can easily mute the original background music by using a video editor. I prefer to use ApowerEdit. Aside from the basic editing function, this tool also enables me to add text, picture, music etc. To my videos in just a few clicks. It really suits beginners like me. Maybe you can also have a try.