If you want to start manipulating a datasource via the Manage Data page you need to make this change in the Report Engine.
First find the datasource you need to manipulate within the report engine.
Once you have found your datasource find the 'Visibility' dropdown:
There are 4 states for a datasource:
Normal : hidden from Managed Data
Visible : visible within Manage Data
Editable : editable within Manage Data
Uploadable : can edit in-situ or upload files
The other side of making a datasource editable is to set the fields within the datasource to also be visible / editable.
Click the 'Fields' button and you will see all the fields within the datasource.
In this view you can set the fields to normal, visible, editable or uploadable.
Once you have made the changes to your datasource you should hit the black arrow at the top of the page to send the changes you have made to the front end: