You can record over an existing audio recording, or a portion of it, replacing the original recording with the new recording. You do this by recording in Replace mode. You can chose one of the following Replace mode behaviors from the Replace pop-up menu:
Region Erase: Erases audio regions from the Tracks area from the start of recording until the end of recording, even if nothing is played.
Region Punch: Erases audio regions only where audio has been recorded.
Content Erase: Does not erase audio regions but the recorded audio inside of them, from the start of recording until the end of recording, even if nothing is played.
Content Punch: Does not erase audio regions but the recorded audio inside of them, only where audio has been recorded.
You can access the Replace pop-up menu by long-clicking (or right-clicking) on the Replace button 
in the control bar or in the Recording preferences window.
Replace recording is only available when Show Advanced Tools is selected in the Advanced preferences pane.
Click the Replace button in the control bar.
Click the Record Enable button in the track header of the track you want to record to.
Move the playhead to the point in the ruler where you want to start recording.
Set the metronome for recording, if desired.
Click the Record button 
in the control bar (or press R) to start recording.
Start singing or play your instrument.
Click the Stop button 
in the control bar (or press the Space bar) to stop recording.
A new audio region is created on top of the existing one in Region Erase or Region Punch modes. The content of the existing region is replaced in Content Erase and Content Punch modes.
The original recording is replaced with the new recording.