You can edit something saved in My Content as follows:

If it’s already scheduled

Visit the Manage Displays screen. You can find the Manage Displays button at the top left of the screen of the Studio page.

or the top left of the Business Settings page:

On the right-hand side of Manage, you can click the pencil icon just above it to edit it in Studio

If it’s not scheduled:

Visit the Studio page and open the "My Content" folder, then drag the studio file onto the canvas.

Once you're happy with your edits, you can save the design or save and publish the design.

Related question: how do i edit something saved in my content?