How to override a function using another plugin in wordpress?

I have a plugin which has a function lets say function1 in class named class1. Now I want to override this function from another plugin such that when I activate the another plugin then the override function should work instead of the function present in the first plugin. How can I do that? I have tried extending that class which has function1 into the another plugin but don't know how to override it and make function2 work instead of function1.

via Chebli Mohamed