Here is a great resource: http://www.thefreedictionary.com/alter http://www.thefreedictionary.com/change http://www.thefreedictionary.com/vary http://www.thefreedictionary.com/switch The explanations there are great.

change = change alter = something will change because of an external cause or factor. "The wind has altered the outside temperature" vary = two or more items are different from each other. "The temperature readings may vary during the day". switch = a radical change, from one position to an other. "The weather has switched from sun to rain".