First, I hope you’re not deep into your edit just yet. I’ll explain why in a moment…
If you have already imported your source material, hilight those clip (or clips) in your bin, and go “Clip (menu) -> Modify -> Audio Channels…”
Chances are you’re going to see “Clip Channel Format” is set to 5.1, and “Number of Audio Clips” is 1. Change this so that Clip Channel Format is mono, and Number of Audio Clips is 6, then hit OK.
OR… change the “Preset” pull-down menu to “Mono” and it should do it automatically for you.
Now, here’s why I said I hope you’re not into your edit yet: changing this setting will NOT change any clips you already have in a sequence. You will instead need to modify the source clip in the bin first and then replace those clips already in your edit with the newly modified clip. Tedious!
If you want all your clips to default to mono upon import, then go to your preference and change the “Default Audio Tracks” to mono. (Note that in older versions of Premiere it’s on the Audio tab, but newer versions have it on the Timeline tab.) Do not be confused: this preference attaches to the clips and how they behave when brought into at track in the timeline; it does not change the behavior of the tracks themselves.
(Later on I will post a reply where I grouse about how bad I think it is that Adobe has made “Use Track” the default setting and how it shafts novice editors.)