To change your object, simply click on the object that you want to change—and on the right panel, there's the Change Object option.
Click Browse, and you will be directed to either the 2D or 3D library to replace it with the object that you want.
However, please note that changing objects are only available for 2D and 3D objects—and you can only change it with the same type. For example, you can change 2D with 2D only, and that also applies to 3D objects.
