I’m not sure, if I understand you correctly. So you have one MIDI track, which contains some MIDI data. You duplicated these data to other MIDI tracks. And now, you want to edit all of these data of all of these MIDI tracks at once? You have to options: A: Use Shared Copy, if you know, you will always need exactly the same data on the copies. If you are using drag-and-drop, while holding Alt, to copy the MIDI part, wold instead of Alt only, the combination of Alt+Shift. This will create the Shared copy. Then, you can edit the data on the one place only, and all shared copies will follow.
B: Select all events, you want to edit. Open any MIDI editor. Via lasso (not just clicking to dedicated event), select the event, you want to edit. All the same events in all parts will be selected. Edit the data.
No problem with this in Cubase at all. Select the MIDI Part in the Project window, and choose Dissolve Part from MIDI menu. In the Dissolve Part dialog, keep the default settings: Separate Channels. Click to OK. New tracks will be created. Every single track contains just the data of one MIDI Channel.