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

Ex Input:
Sentence: 'people watching a kite being flown with a sky background'. Replace the letter 'a' with 'b' in the sentence.

Ex Output:
people wbtching b kite being flown with b sky bbckground

Ex Input:
Sentence: 'a couple walking down a sidewalk next to some graffiti'. Replace the letter 'a' with 'y' in the sentence.

Ex Output:
y couple wylking down y sidewylk next to some gryffiti

Ex Input:
Sentence: 'a group of people on motorcycles riding through a rock tunnel and a sign says falling rock'. Replace the letter 'm' with 'x' in the sentence.

Ex Output:
a group of people on xotorcycles riding through a rock tunnel and a sign says falling rock