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

Example Input: Sentence: 'a person on a snow board flying through the air'. Replace the letter 'f' with 'n' in the sentence.
Example Output: a person on a snow board nlying through the air

Example Input: Sentence: 'a couple sits on a bench in the countryside'. Replace the letter 'y' with 'k' in the sentence.
Example Output: a couple sits on a bench in the countrkside

Example Input: Sentence: 'a woman riding skis holding ski poles on a snow covered slope'. Replace the letter 'a' with 'w' in the sentence.
Example Output:
w womwn riding skis holding ski poles on w snow covered slope