If you rented it directly on your iPhone then you can't - from here :
- If you download a rented movie on your computer: You can transfer it to a device such as your Apple TV (1st generation), iPhone, iPad, or iPod if it’s a standard-definition film (movies in HD can only be watched on your computer, iPad, iPhone 4 or later, iPod touch (4th generation or later), or Apple TV). Once you move the movie from your computer to a device, the movie will disappear from your computer's iTunes library. You can move the movie between devices as many times as you wish during the rental period, but the movie can only exist on one device at a time.
- If you download a rented movie on your iPhone 4 or later, iPad, iPod touch (4th generation or later), or Apple TV: It is not transferable to any other device or computer.