- Click artboard tool, and up above in controls change your width and height to 720 px x 480 px.
- Select all and scale you artwork. You may want to do this visually, or manually change in the transform panel 1080 to 720 or instead 1080 to 720, or both if you wish to distort. You will want to pay attention to your reference point to be in the centerer, or maybe another option. This step depends on who your artwork is built and what is easier for you.
for future versions option click your scale tool, and you can write down this number for future reference, and just click ok. You may even be able to record an action if you plan on doing this often in the future, but you should first become intimately familiar with scaling and artboards.