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

Example Input: Sentence: 'a couple walking down a sidewalk next to some graffiti'. Replace the letter 'a' with 'y' in the sentence.
Example Output: y couple wylking down y sidewylk next to some gryffiti

Example Input: Sentence: 'a double decker bus stopped outside a station on a street'. Replace the letter 'c' with 'y' in the sentence.
Example Output: a double deyker bus stopped outside a station on a street

Example Input: Sentence: 'a colorful filled pastry in a white napkin'. Replace the letter 'c' with 's' in the sentence.
Example Output:
a solorful filled pastry in a white napkin