How run custom update code in gridview?

I have a gridview with several fields.  The user should be able to click 'edit', update several fields, then click 'update'.  I want to run CUSTOM code for the update command.  

How can I do this?  I suppose I would check the e.commandname in the rowcommand event, but I don't know how to access the controls that contains the new values.

Any code samples?

Thanks.