How do you fix a garage door motion sensor?

Most garage doors have little motion sensors toward the bottom the door that sense movement or things in the way of the door as it closes. my sensors seem to be not up to bar and i would like to figure out how to fix this. Recently they have not been sensing stuff in my garage such as recyclable such as plastic bottles. Now i wouldn't be too worried about it until yesterday when I closed the garage door and went in the house. i could hear my cat meowing and i opened the door leading out to my garage to see that my cat was pinned between the garage door and the floor. my cat is fine now but i would like to fix this problem so that it doesn't happen again.