In some cases, you won’t be able to.
The viva is not a question and answer session or a quiz. Some questions won’t have memorisable facts that you can serve up to your examiners; instead, you will have to offer another contribution, a response – a detail, an opinion, an argument, a feeling, a hunch, a question – in order to keep the discussion moving forward.
Your response may not be the entirety of everything you want to say. It may be that you have to pause and reflect first, make notes, stand up and draw something, or ask for clarification.
You may not be able to answer a question, but after a little thought you will always be able to respond.
If the question is difficult, then you owe it to yourself to think a little more, pause a little longer, take a little more care, even ask for a little more, so that you can respond as best as you possibly can. That response could be an answer (truth, or an argument with a lot of evidence), but it could be something else that is just as much what your examiners could be looking for.
Every question, not just the difficult ones, deserves a little time, a little space, a little thought in order for you to give your best response.