Media library

Hi there, it seems I can’t modify the image with the same name in media library. After I posted an image, I found the image was too small. So I enlarged it ans reloaded the file into the media library (I deleted the previous image permanently). However the image didn’t look any different on the page. I had to change the file name and the name in the code. This time the image looked fine. I am not sure if this is because where I did wrong or it is an issue. I want to edit image and repeat using the same file. Please help. Thank you:-)!