From your account Statistics page, click the EDIT button next to the article you wish to edit.
From the edit page, go to the Add Capsule block on the top of the right, column. Click the type of capsule you want to add. It will flash a little LOADING flag very briefly - sometime so fast you can't see it. Now look to the very bottom of your hub and you will find the new capsule. Click the EDIT button on the capsule to add content. SAVE the capsule when done. Use the green arrows to move the new capsule into place (or use the drag & drop utility in the right column.
If the hub has been published, click UPDATE. If not published SAVE CHANGES or PUBLISH your hub.