If you have a mathematical expression entered into Google Docs, you can edit it by simply selecting the inserted mathematical equation, and clicking the "edit math" button, which will then populate the input box with the selected math:

For complex expressions which cannot be done in the Simple Tab, clicking “edit math” will automatically switch to the Advanced Tab, and populate with the given expression:

This works in both Simple and Advanced. When using the Advanced tab, the dialogue box will populate, as well as the LaTeX editor text box which will populate with the LaTeX code: