You will first need to unlink the video and audio in premiere. then slice up your audio with the razor tool. Delete the slice you want to lose. Cut a paste a slice of background noise from elsewhere into the gap.
Audio does nto work the same as video. Audio on a higher layer does NOT replace audio on a lower level as it effectively does for video. The two get added together so you need to delete the original bit.
After pasting the background noise into the gap you might want to apply a cross fade so you do not get a sudden change of background noise.
This cut and paste method is a lot quicker than my Audition route but will only work if there is no background noise/sound you want to keep. If it were a pop or crackle in music then the audition route would likely be better. For this job though, it's probably overkill - unless you want to practise of course
