Correct the unit at the point where the mistake occurred and restart only the affected part of the calculation.

Restarting everything wastes time and can make a small error feel larger than it is. The interviewer mainly needs to see that you identify and fix it correctly.

If your formula is right but one input was entered as 500 instead of 0.5 million, state the unit correction and recompute from that step.

Keep units written beside numbers throughout the calculation so you can spot mismatches earlier. If useful, this guide on case interview mental math gives more detail.

I'd fix the unit from the mistake point, not restart everything.