Are you using Connect or the Web Interface?
And then are you using the Web App or the Field App to fill in those surveys?
There is no option in the Web Interface to update "description" because it isn't something that displays in the Web App. But, if you are using the Field App, I'd recommend using Connect to build the survey.
EDIT: There is a "description" in the Web Interface, but it isn't quite what you descried (see below). That description should be visible at the top of the page, and you can just click on it to edit.