The LM2576HV operates when the feedback signal to the FB pin is around 1.23 volts so, use the digipot from FB down to ground and have an upper resistor to the output voltage of suitable value.

This will allow you to get up to whatever voltage you need but, it won't allow you to turn off the output below 1.23 volts. However, you can fudge this by injecting a current into the FB node.

One thing you have to watch for is the increased capacitance on the FB node that the digipot brings and you'll probably need to compensate for this with a capacitor across your upper resistor.

The LM2576HV regulates when the feedback signal to pin 4 is around 1.23 volts so, use the digipot from FB down to ground and have an upper resistor to the output voltage of suitable value: -

This will allow you to get up to whatever voltage you need but, it won't allow you to turn off the output below 1.23 volts. However, you can fudge this by injecting a current into the FB node.

One thing you have to watch for is the increased capacitance on the FB node that the digipot brings and you'll probably need to compensate for this with a capacitor across your upper resistor.