When creating a new layer, what's the step to later modify the table of contents in ArcGIS online?
within ArcGIS Online specifically? Copy and save layers—ArcGIS Online Help | ArcGIS
or are you looking for Best practices for using layers in maps—ArcGIS Online Help | ArcGIS
Are you referring to editing the attribute table in ArcGIS Online?
It might be best to look over how to edit features in AGOL: Edit features—ArcGIS Online Help | ArcGIS