Changes made to elements are saved automatically. You can track this in the top left corner of the Editor.
When you place a new element, or change an existing element, you won't see anything at first. This means your changes will be lost if you leave the page, and your web browser will give you a warning if you try to close the page.
To make sure your changes are saved, click anywhere outside the newly-placed or changed element. You'll see a check mark in the top left corner of the Editor, as shown below, which means your changes have been saved.