Free software to synchronize audio and video recorded at the same time
What you need is called a non-linear video editor or NLE.  Basically you can load in the files and then you put them on a "timeline".  Each audio track and video track goes on a horizontal track.  You move the clips right to left on the timeline to synchronize them and once you have them synchronized, you can cut out portions of the top most clip that you don't want and the bottom clip will take its place since it is under the top clip.  You can also add in transitions to switch between the clips.

There is a lot more to video editing as you get in to it, but that should give you the primer you need to get up and running.  Take a look at the duplicate post for suggestions of some NLE software you can use for free.