Changing Instruments after part is written

So I wrote a clarinet duet but mistakenly had the first part as Tenor Sax. I want to change the Tenor Sax line to Clarinet, but don't know how. I've tried a couple diferent ways, I changed the text from Tenor Sax to Clarinet, but that didn't do anything. I tried a couple other ways to no avail. I really want it as clarinet because otherwise the midi sounds really messed up.