How to edit a story
For this you will need the Graphql id of the story (see How to get a Story).
This example demonstrates how to change the file (in the Main asset) to another one, the variables are used in the same way as in How to post a story with a file.
graphqlmutation storyEditExample($input: StoryEditInput!) {storyEdit(input: $input) {story {slugversion {titlemainAsset {__typename... on FileAsset {downloadUrlembedmimetypefilesize}}}}}}
variables{"input": {"id": "STORY_ID","addedAssets": [{"type": "FILE","file": ".eJw...xMWA","uuid": "GENERATED_UUID"}],"mainAsset": "GENERATED_UUID","title": "My story with a new file as main asset"}}