Improving the characters is crucial. Give them more depth, motives, and personalities. Let them grow and change throughout the story. Also, work on the writing style. Use language that is appropriate for the story and the intended audience. Avoid being too wordy or too simplistic. Another important aspect is to make sure the story has a clear theme or message that the reader can understand and relate to.
To improve'really bad short stories', start by getting feedback from others. They might see problems that you don't. Then, read a lot of good short stories to understand what makes them work. Look at how the best authors create engaging plots, develop characters, and use language effectively. When revising your own story, be ruthless. Cut out any parts that don't add to the story. And always keep in mind the overall mood and tone you want to create.
One way is to focus on the plot. Make sure it has a clear structure, with a definite start, some sort of conflict in the middle, and a resolution at the end.