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

Example Input: Sentence: 'a train traveling through a dark tunnel next to green brush.'. Replace the letter 'r' with 'e' in the sentence.
Example Output: a teain teaveling theough a daek tunnel next to geeen beush.

Example Input: Sentence: 'batter is twisted up as he took a swing at the baseball'. Replace the letter 'h' with 'i' in the sentence.
Example Output: batter is twisted up as ie took a swing at tie baseball

Example Input: Sentence: 'a man sitting on a bench next to a fire hydrant'. Replace the letter 'f' with 'u' in the sentence.
Example Output:
a man sitting on a bench next to a uire hydrant