Once the object is in the diagram displaying area, you can select it to move it or make changes.
Selecting an object
To select an object, use the Pointer tool from the Palette toolbar, then click anywhere inside the object. The solid borders of the object will turn into the dashed ones.
Selecting multiple objects
To select multiple objects, use the Pointer tool from the Palette toolbar, then click the left mouse button and drag the selection rubber-band so that the rubber-band box encompasses the objects you want to select. Once all objects are within the rubber-band, release the mouse button.
To add objects to the list of the already selected ones, use the Pointer tool from the Palette toolbar, then click anywhere inside the object holding the Shift button. If the object has already been selected, it will be deselected.
Please note:
If you move mouse cursor from left to right when selecting only objects that are completely inside rubber-band will be selected. If you move mouse cursor from right to left than objects will be selected even if they partially covered by rubber-band.
Selecting all objects
You can also select all objects in the diagram. For that purpose, click on Select All from the right-click shortcut menu. Another way to select all objects is to use the Edit | Select All menu item or press Ctrl-A.
Deselecting objects
To deselect a selected object, click anywhere outside the selected object(s).
Moving objects
Once you have selected objects, you can change their location in the diagram. Just click within the selection with Pointer tool and drag the objects to the new location while holding down the left mouse button.
If Snap to Grid option set in Diagram Display Preferences them objects will snap to grid while moving.
See also:
Diagram: Using Snapping Grid | Diagram Display Preferences