You will only be able to edit content that you author. To modify an existing shared resource, open Commons in the global navigation. Search for the appropriate resource and then click on the name of it to open it. Once open you should see an Edit Resource button. Click the Edit Resource button. This will allow you to edit the "details" of your resource: sharing, tags, description, etc.
To edit a learning activity, you will need to re-share the modified resource. The modified resource will take the place of the preexisting resource.