as is said, you need to be clear the difference between midi channel used by mother.pd, which are controlled by midi-setup, ,and those hard coded in the patch.
I suspect you need to look at both to get the result you are looking for!
unfortunately this likely depends on the patch, as there are different ways to achieve similar results.
to start, look in pd at help->list of objects, look for the midi objects, so you know what to look for in the patch.
most likely the patch is using notein/ctlin which take the midi channel as the first parameter, so then its a simple matter of changing that
however, its also possible to do it differently e.g. send in the channel dynamically, of receive all midi, and the filter out data thats not on the channel you are interested in… if its doing that then you’ll need to look in the patch to see whats is up to.
but as i said, most simple patches will simply do something like [notein 3] to take notes on channel 3
(Id recommend having a look at the pd help text for the midi objects too, the examples are quite good)
as often, when trying to describe ‘how to patch’ , the written explanation sounds much more complicated than it really is, if you open the patch, it will probably be pretty simple (now, you know to look for the midi objects).
pd is designed to be easy to code, so don’t be scared , just dive in , try different things… the more you ‘play’ the more you’ll learn, and then find it trivial to do for the next patch you look at.
tip: save a copy of the original patch, so that when you start making changes, if it goes wrong, you can easily go back to the original.