Place the video on Video 1 on your timeline. Right-click on it and select Delete Audio.
Then hold down your Ctrl key (to prevent ripplling) and drag your new audio to Audio 1.
I assume you used a clapper or some other audio/video cue so that you can sync the two tracks. Otherwise, it's just trial error to get them in sync.
Once you have followed Steve's steps, to adjust the sync perfectly, see this article: http://forums.adobe.com/thread/436751?tstart=120
Now, some users have found that their cameras, and their separate audio recorders do not sync 100%. With today's technology, they certainly should, but camera and recorder companies do not see to care, so leave the task to the editor. One user found that his Zoom recorder was about 0.06% slower than his Panasonic cameras, by trial and error, and critical listening. This was a constant sync error. To correct, he changes the speed of the Audio 1.04%, with Maintain Pitch checked - then it is always perfect. If you find that you cannot maintain perfect sync, explore speed settings for the Audio. If the recorder is a high-quality, the OOS (Out Of Sync) should be constant, and when you find the correction, it should be the same for all projects, so the trial and error should pay dividends.