Before editing an element you first need to select it. To select an element simply click it with the left mouse button in the Editor's viewport
or click on it in the hierarchy panel.
If several elements are overlapping you can select the one in the back by holding the Ctrl key and using left mouse click.
Once you select the element you can edit its position, size, rotation as well as other properties.
Selecting Multiple Elements
You can also select multiple scene elements by holding and dragging the left mouse button in the Editor's viewport.
You can change the zoom level of the Editor's viewport with the mouse scroll and move the currently visible part of the viewport by holding the ALT keyboard button
You can use drag and drop to position the element in the Editor's viewport. You can also set the position of the element in the properties panel. You can move multiple selected elements.
You can change the size of a element in the Editor's viewport simply by drag and dropping the square manipulators that appear around the element.
You can also set the size of the element in the properties panel.
Once you select an element by default it goes in resize mode.
If you want to rotate an element simply click again on the element and it will go in rotate mode. Then you can use the rotating arrows handles to rotate to element in the viewport. You can also set the rotation of the element in the properties panel.
You can edit various other properties of an element in the Properties panel.
You can delete an element by clicking on the "Delete" button in the properties panel (note you first need to have the element selected). You can also delete an element by pressing the "Delete" keyboard key.