Hi Randy,
This should do it.
fs = gis.content.get([the itemID])
fs.update(item_properties = {'accessInformation': " Micheal, Catherine, Amanda"})
Other parameters that you can update are here: https://developers.arcgis.com/python/api-reference/arcgis.gis.toc.html#item, search for "update(item_properties=None, data=None, thumbnail=None, metadata=None)¶"
Just make sure the account you're signed in as in Pro is the owner of the item in AGO.