Different ways to digitally control an adjustable SMPS
The feedback pin is expecting a DC error voltage, with some usual stuff (ripple, noise, etc.) riding on it. The analog voltage loop is bandwidth-limited so that only useful information is used to determine the duty cycle of the converter.

The easiest way is to use a DAC output and a series resistor to either sink or source amount of current out of / into the FB node. The size of the injection resistor will determine the adjustment range. The FB reference voltage is 1.23V, so as long as the DAC can go above and below that reference, you can control the voltage both up and down.

This is the digital equivalent of having the bottom resistor adjustable.