To change the Comment shape:
1. Select the cell containing the Comment and press Shift+F2 to edit the cell Comment or press Shift+F10 (or right-click) and select Show Comment from the shortcut menu.
2. Select the edge of the Comment so that it is surrounded by dots, not slashes.
3. From the Drawing toolbar, click Draw, select Change AutoShape, and then select any shape.
Screenshot // Changing the Cell Comment Shape