There are lots of options. If you have a pretty good computer can use kdenlive on Linux or windows movie maker if on windows I think there is one built into Mac os as well. Even the most basic video editor will allow you to replace the audio, with kdenlive or Adobe premiere or something else that is more complex you can have finer control on mixing multiple audio tracks and controlling transitions between clips.
Barring any of that you can just edit a YouTube video after you upload and can add audio that way as well (they have copyright free tracks you can search and add on).