How to rob a iPhone 5 off someone on the streets?

So my mom is taking my phone tomorrow and i need to get another iPhone 5 but i can't afford one and i have a pocket knife so how do i rob someone? Obviously i don't intend on using it or stabbing someone for it but i have just for threatening purposes so how can i do it?

Simple. Get a job, work for it, you will feel a lot better about yourself in the long run, it's not now you gotta worry about, it's the cops and looking back on it, and the guilt