There are multiple ways to change associated Epic of User Story.
- Using Individual User Story
- Planning Board
You can use Planning Board to move multiple User Stories in different backlogs.
- Go to Planning board
- Select your desired Backlog, Project and Sprint.
- Use the drag and drop functionality to move User Stories from one Backlog to desired Epic.
You can edit individual User Story and assign desired Epic from Epic field. You can use Search option in the Epic dropdown field to quickly find your desired Epic.