For assets placed in the scene, select the asset and press the W key to display the arrow gizmo in the center of the asset, and move it by dragging the arrow.
The Arrow Gizmo
The arrow gizmo has three differently colored arrows that represent the directions in which you can place things in 3D space:
Red: X-Axis (Horizontal)
Green: Y-Axis (Vertical)
Blue: Z-Axis (Depth)
For example, by dragging the red arrow you can move an asset left and right in the horizontal direction. The green arrow moves assets up and down in the vertical direction. By clicking the box at the center of the gizmo, you can move assets in the plane represented by the color of the side you click.