Add a stave for cello. Copy the notes of the English Horn and paste them into the cello stave. Then you may remove the horn stave using the instruments dialogue with 'i'.
Using right mouse button on the horn stave and choose in stave/part properties a new instrument - the cello.
Comments
There are at least two methods (MS 3.6.2):
Add a stave for cello. Copy the notes of the English Horn and paste them into the cello stave. Then you may remove the horn stave using the instruments dialogue with 'i'.
Using right mouse button on the horn stave and choose in stave/part properties a new instrument - the cello.