First, it's important to note that you can convert a Multichannel track into separate mono tracks, but you can not do the reverse.
You can convert a Multichannel track(s) into separate mono tracks in two ways. The first method allows you to convert specific tracks, individually, while the second method will convert all the tracks in the Sequence.
This first method will let you convert individual Multichannel tracks into separate Mono tracks.
Duplicate the Sequence(s) before converting tracks. To do this, first select the Sequence(s) in the bin. Then go to Edit Menu | Duplicate. Lastly, amend the Sequence name(s) to indicate which is Multichannnel and which is Mono.
Make sure the correct Sequence is loaded in the Record Monitor/Timeline (that is, the one that is to have a Multichannel track converted to mono tracks).
In the Timeline, right-click on the Track Selector of the Multichannel track you want to convert.