This page will explain how to change a basemap for a web map in ArcGIS Online (AGOL)
In AGOL, navigate to your Group page
Press the "Content" tab in your Group page
Find and select your Collector Map (Organization Name)
Press the "Open in Map Viewer" button
See the Map Viewer with the Collector Map (Organization Name)
Press the "Basemap" button
Choose your desired Basemap Note see the difference in the two Basemaps
Press the "Save" button