Once a field name is set and saved in its source
layer, it cannot be changed. However, the display name of the field name can be
changed in ArcGIS Online or Portal for ArcGIS. A display name is the field name
displayed on the hosted feature layer's attribute table. This video describes
the workaround by adding a new field and copying the field values from the
intended field to the new field. Item 31571-2024