1. Open the project in Maya and select the cube whose outline you want to round. You can use the selection tool or click the cube directly to select it.

2. Enter AttributeEditor, which is usually located on the right. Click the selected cube, and the property editor will display the detailed properties of the cube.

3. In the Attribute Editor, you can find the graphic option and click it.

4. In the pop-up menu, select Edit Grid.

5. In the menu of editing grid, you will find an option named "Bevel", click it.

6. Next, you need to adjust the chamfering parameters to make its outline more rounded. Adjusting the value of Bevel Level can make the profile smoother, and adjusting the value of Bevel Thickness can make the profile more round. The specific value can be adjusted according to your needs.

7. Finally, click the "Apply" button in the property editor to apply your changes, and then click the "OK" button to return to the main interface.

Now, you have successfully made the outline of the cube more round.