I'm creating a task flow (for the first time) for a mobile app. This app contains an edit/update page where the user can choose to edit/update one, several, or all the fields in a form. What would be the best way to show this in a task flow?
I think you'll find that most types of workflows require a number of different guides/cues to cater for different types of user behaviour, at least for new apps without a standard or convention that people are used to.
The convention is to provide a brief piece of text right at the beginning of the form to tell them that all of the fields must be filled (perhaps the first time they enter the workflow) and that it is optionally editable (perhaps if there are pre-filled fields).
Then you can use either symbols or icons for each field that is optionally editable (which is going to look cluttered if all of them have this property), but at least it means that if you introduce fields that are not editable then you have a design convention which you simply reverse to indicate non-editable fields.
However, in your case it seems like you are switching the states of fields on a page from view only to an edit/update state, rather than having only one view of the page where the user has to individually trigger the edit or update function on the field. So if you provide a logical transition from the view state to the edit/update state (e.g. change fields from labels to input fields or from a disabled to enabled state) then the user will understand this to be editable.
If yours are like mine, the primary users of workflow documents are developers. So I'd recommend you create a first draft of the workflow that includes textual notes to describe interactions and rules. Show it to the devs and see if they need additional information. Ask how the info should be displayed so they can best understand it. Document design is an iterative process as much as product design.
Your findings on this project will inform how you do future workflows, wireframes, and sitemaps.