This depends on how underprepared you actually are and whether a reschedule is realistically available. There is a big difference between feeling nervous because you are not perfect and knowing that you still have major gaps in structure, math, or basic case mechanics.

If your performance is already interview-capable but inconsistent, I would be cautious about delaying solely to chase confidence. Most candidates never feel completely ready. Extra time can help, but it can also turn into another two weeks of anxiety without fundamentally changing the result.

If you are genuinely not ready, a reasonable reschedule request may be better than using an opportunity before you can compete. The key is to ask professionally and understand that the firm may or may not be able to accommodate the change.

I would make the decision based on evidence. Do two or three realistic mock interviews with strong partners and ask whether you are currently passing. That is much more useful than deciding based on how nervous you feel.

If you are truly not passing mocks, I would ask to move it. If you are just anxious, I would probably go.

How underprepared are we talking? Like bad math and structure, or just not enough reps?

I pushed mine 10 days and it was absolutely the right call.

Tbh more time only helps if you know exactly what you are fixing.

I would not assume they will let you move it, so be ready for either answer.