- Select Feature Layer from Content in Portal.
- Select Data menu on the right side of Overview.
- Select Fields.
- You may want to edit the type by clicking the field name but you cannot save it in ArcGIS Portal 10.8.
- You can delete the existing layer and upload a new CSV if you want to change type. You can specify the correct type after you finish uploading CSV data.
If the type of time field is String, time-based selection in Dashboard will not work. It should be Date.