In this task, you need to replace a letter in the sentence with another given letter.
Example: Sentence: 'group of motorcycle motorists trying to merge into traffic'. Replace the letter 'o' with 'n' in the sentence.
Example solution: grnup nf mntnrcycle mntnrists trying tn merge intn traffic
Example explanation: The letter 'o' has been replaced in the letter 'n'.

Problem: Sentence: 'a blue street sign with the words merton st. is next to a white piece of artwork'. Replace the letter 'b' with 'f' in the sentence.
Solution: a flue street sign with the words merton st. is next to a white piece of artwork