Is there a way to change the default instrument without external midi ?

A pattern editor is not associated directly to a MIDI program, only to a MIDI channel. But each MIDI channel can have or not its own instrument. By default, they do not have any.

There is in each pattern editor an instrument menu (the "ins." menu) from which you can choose which MIDI intrument is to be associated to the pattern channel.

When an instrument is associated to a channel, you can see it featured directly in the channel menu. The corresponding MIDI program change message will be added at the beginning of each pattern on that channel.

Does this answer your question?

Do not forget to go through the built-in help topics, they are supposed to be comprehensive (and comprehensible...)

Best,

Stef