A typical approach is to control the feedback network adding a series resistor to the ADJ pin (while keeping a resistive divider to Vout), and driving this resistor with a voltage DAC.
(Vref is the internal reference of the regulator, iADJ is the typical current drawn from the ADJ pin, which can often be neglected.)
This approach is usable with linear and (usually) switching regulators as well.
Select resistors values appropriately. The control voltage can be restricted to a low voltage.
One additional benefit is that if Vctrl is or can be high impedance at any point (for instance, upon power-on), the output voltage will still be controlled via the resistive divider, and can be set to a predefined value.
