As far as I can tell there is no way to move a field when editing attribute table in AGOL's Data tab.
Having this option would be very helpful and remove the need to Overwrite the entire layer.
AGOL's Data tab:
https://doc.arcgis.com/en/arcgis-online/manage-data/add-or-delete-fields.htm
ET Geowizards has an "Order Fields" tool that performs this function:
https://www.ian-ko.com/ET_GeoWizards/ETGW12UG_WEB/etgw/orderFields.htm
