Note the beginning measure of the instrument change, and the original instrument.
Choose Window > ScoreManager.
Click the arrow+ to the left of the Instrument Name of the original instrument (the one beginning on measure 1).
Identify the instrument by referring to its Name and the Start Meas. column, then click the "X" icon to the right to delete it. (If the Start column is not visible, choose Customize View > Start Measure to show it)