Since you created "My Test Card" you are its editor and have the power to edit it. Click the "Edit" button on the card to get back into the editor.
Click "New Text Chunk" to create a new text chunk. Now enter "Discovered that Elvis is Alive and living under the sea with the Loch Ness Monster".
If this is too minor an update to notify followers then you can check the "minor update" box to make the update silent. However this update seems pretty big, so let's leave that box unchecked.
Since this is a major update, you need to provide a summary of the change that can be shown in the Updates Inbox for followers. Enter "Elvis is Alive", then click "Update Card".
Next: Link to Cards