The event structure does not provide this information by default. You need to manually compare the old and new values of the array in order to get the index of the modified element. See this example for how to compare the elements:
Note: This image is a LabVIEW snippet, which includes LabVIEW code that you can reuse in your project. To use a snippet, right-click the image, save it to your computer, and drag the file onto your LabVIEW diagram.
This code was created for an array of buttons, but works the same way for all common datatypes.