Normally, you need to select an object to change its properties.
To select objects using the mouse, activate the Selection tool (usually activated by default).
To select a single object, click on in it.
To select multiple objects, click on each while holding down the Shift key. Or, select several objects with a frame by clicking and holding down the left mouse button and encompassing these objects.
To select all objects in the active layer of the current page, choose Edit > Select All (Cmd-A).
To deselect an object, click on any free space inside or outside the page.