You can't just voluntarily cancel car insurance without either getting new insurance to replace it or providing proof you no longer have the car or are no longer driving it. Best for you would be to get new insurance from a new company based on accurantly disclosing that you drive for Uber. Bear in mind that if you get into an accident while driving for Uber, if you did not disclose that you do this to the insurer, the insurer can likely refuse to pay on the grounds you obtained insurance under false pretenses. To have coverage you can rely, you need to disclose how you use your car.