Here are the answers for each sentence using "must" or "mustn't":

1. You **must** be fit if you want to become a champion. (must)
2. Mum gave me a letter to post. I **mustn't** forget to post it. (mustn't)
3. Whatever you do, you **mustn't** touch that switch. It's very dangerous. (mustn't)
4. My sister **must** go to Spain for her studies next year. (must)
5. The doctor told me that I **must** stay in bed for a week. (must)
6. You **mustn't** unfasten your seat belt during take-off and landing. (mustn't)

Feel free to ask if you have more questions!