Exporting a 2 USB mic recording from stereo to mono

I am new to podcasting. We are using 2 USB microphones , set up as an aggregate device, to record 2 different people in Garageband. This records one mic on the right and the other mic on the left (in stereo). I would like to export that stereo track as a mono track so it doesn't play each voice in separate speakers when it's played back by people who listen to it. Any good ( see easy) way to do this?

Someone suggested I could export it in mono when I "share" the file to disk. They said I should choose "custom" when I try to save it to disk but I'm not seeing that option.