After adding text within a shape (or a text box) in PowerPoint, you can also perform basic edits like adding, deleting, or replacing existing text. Other than the basic text editing you can even control how your text is aligned within the shape. To do that you need to access these options within the Format Shape Task Pane.
To learn more, choose your version of PowerPoint. If we do not have a tutorial for your version of PowerPoint, explore the version closest to the one you use.