Detailed Instructions: In this task, you need to replace a letter in the sentence with another given letter.
Q: Sentence: 'a knife that has been used to cut into metal tin'. Replace the letter 'a' with 'u' in the sentence.
A:

Show your work and conclude with the exact phrasing "Therefore, the final answer is [answer]." where [answer] is just the final number or expression that solves the problem.
user
u knife thut hus been used to cut into metul tin