1. You can easily make your existing page into a sub-page to another page of your choice.
2. Go to menus under appearance.
3. First, move the page you wanted to make as a sub-page/sub-item below another page where you wanted it to become it's sub-page/sub-item. From the menu structure point your cursor to the page you wanted to become a sub-page/sub-item, left click and hold then move it a bit to the right and then release/drop. You will see that the page is now marked as sub item. To put it back as a page just click and hold your mouse then move the sub-page/sub-item to the left then release/drop.
4. Once you've done your changes, click the save button.