How can I whiten the back of my front teeth?

If you did zoom or laser bleaching usually the front of the teeth will be whitened ! In order for the whole tooth be whitened see a knowledgeable dentist to evaluate, first do a cleaning and get bleaching done with take home trays! 

If you get custom made Whitening trays from your dentist, you can place the whitening gel in the back too.
But since these surfaces are not visible, it's not recommended because we consider it "waste of the whitening gel"

With my whitening clients, if there is particular concern involving specific teeth then I create the whitening trays with a small reservoir at the front and back of so that the bleach can pool and act on both sides of the teeth. I find this is very effective, even if the teeth have been root canal treated. I hope this helps.