Since many are just getting started with Web GIS, one may not remember how to find an existing web map app and then continue editing it (to further add configuration, etc).
This short video shows you where in ArcGIS Online you do this for Web App Builder.
The basic steps are:
- login to ArcGIS
- go to My Content
- Find the “Web Map Application”
- Left click and choose Edit Application
- NOTE: For “Configurable Apps”
- Right-click on the web app
- Choose Item Details
- Choose Configure App. See the screen shot below.

This short video shows how to edit an existing web map app.