How do I change file associations?

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http://forums.androidcentral.com – Someone sent me an excel spreadsheet today and I tried to open it on my phone. I chose to open it with Quick Office and I chose "Always" and it won't open the file. I think the other choice was Spreadsheet viewer. I also have the app Documents To Go. Is there any way to change the file association once you've chosen to use a certain app "Always?" Thanks! (HowTos)