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

Example Input: Sentence: 'a couple of giraffe standing next to each other'. Replace the letter 'i' with 'q' in the sentence.
Example Output: a couple of gqraffe standqng next to each other

Example Input: Sentence: 'a man walking a horse on a dirt track'. Replace the letter 't' with 's' in the sentence.
Example Output: a man walking a horse on a dirs srack

Example Input: Sentence: 'a person on skis going down a hill with trees in background'. Replace the letter 'l' with 's' in the sentence.
Example Output:
a person on skis going down a hiss with trees in background