Are those audio tracks or software instrument tracks? For audio track, click-hold the input format button (to the left of the input slot that is at the top of the channel strip) and choose Mono.
For software instrument track, insert a mono instrument, or click-hold the right part of the instrument slot and see if you can choose a "Mono" format for that instrument. If the instrument isn't available in mono, then you can insert a mono plug-in in the first audio fx slot below the instrument: hold down Option as you click to insert the plug-in and choose for example Utility > Gain. Just inserting the mono plug-in will turn the audio signal at that poin in the signal flow into a mono signal.
Perfect. Checked in the Multimeter and it definitely turns it into mono.
Cheers