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

Example Input: Sentence: 'a blue and white train is coming down some tracks'. Replace the letter 'b' with 'u' in the sentence.
Example Output: a ulue and white train is coming down some tracks

Example Input: Sentence: 'a man walking his dog near a sandy area'. Replace the letter 'i' with 'g' in the sentence.
Example Output: a man walkgng hgs dog near a sandy area

Example Input: Sentence: 'a red and black passenger train sitting on the track'. Replace the letter 'n' with 'd' in the sentence.
Example Output:
a red add black passedger traid sittidg od the track