In this task, you need to replace a letter in the sentence with another given letter.

Example Input: Sentence: 'a close up of a train on train tracks at night'. Replace the letter 'n' with 'm' in the sentence.
Example Output: a close up of a traim om traim tracks at might

Example Input: Sentence: 'a mobile phone and a set of keys on a key chain'. Replace the letter 's' with 'j' in the sentence.
Example Output: a mobile phone and a jet of keyj on a key chain

Example Input: Sentence: 'a red double decker bus on a busy street'. Replace the letter 'b' with 'x' in the sentence.
Example Output:
a red douxle decker xus on a xusy street