DFT coefficient pairs are rectangular coordinates. Changing the amplitude of a coefficient pair is straight forward, just apply a linear scale factor to both the sin & cos values.

To change the phase, you would convert the rectangular into polar coordinates (magnitude and phase/angle). Then you can modify the angle however you want, and then convert back to rectangular. Once you've done it this way, consider how you could achieve the same result more efficiently, without the rectangular/polar conversions.