This is going to be a slight over-simplification, but it's basically a directional thing according to the perspective of the speaker and the person addressed by the speaker. If you are upstairs and the other person is downstairs, you would say bring it upstairs, as bring is used to denote something coming towards the speaker. Take it upstairs would imply that both you and the other person are downstairs, as take is used to denote movement away from the speaker. There are other, more subtle, nuances which can change things a little but I'd be here all day . . . that's basically the info you're after.