On the top-right you have the source and destination instrument and then a little lower to the left you have the Swap, Remap and Delete buttons.
Select the target instrument as the instrument you want to change (in the selected area) and the source instrument you want it to change to.
Then click “Remap” to have the instrument changed.
If you select “Swap” all instruments in the selected area that currently ARE the destination instrument will be changed to the source instrument.
“Delete” will only remove the notes that use the source instrument number.
So with this button you can remove a specific instrument from a selected range without affecting the other instruments in there.