Actually, several ways:

1. Click SOLOIST and select from the drop-down list.
2. Use SOLOIST > SELECT MIDI INSTRUMENT > best RTs. You can can customize your soloist instrument.

Also, click on the track that you want to change instruments (far right side in Windows version. The small white down arrowhead) that will display a list of instruments, as Jim suggested. There will be a similar procedure in the Mac version.

Don