Step by step guide

1. Select Feature Layer from Content in Portal.
2. Select Data menu on the right side of Overview.
3. Select Fields.
4. You may want to edit the type by clicking the field name but you cannot save it in ArcGIS Portal 10.8.
5. 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.

Note: If the type of time field is String, time-based selection in Dashboard will not work. It should be Date.