I think you should probably be asking your question to Mr. Marshall, but …
The H9 expects a MIDI Program Change message to change presets (patches ?). Many pedals will send this.
I don't know what is required to change your amp's channel – my guess is that this will be a MIDI CC (Continuous Controller). If this is the case, you will need one of the more sophisticated controllers to send this at the same time as the Program Change above. These are beyond the remit of this Forum.
Hopefully some of our users have faced this situation and can give advice.