I found this description:
Adding fields to cards
You can configure cards on a board to display up to three additional fields. The fields can be different for the Backlog and Active sprints, if you are using a Scrum board.
To add fields to cards:
- Click the Jira icon > Projects > then select the relevant project.
- Go to your Scrum backlog, Active sprints, Kanban backlog (if enabled), or Kanban board.
- Go to ••• > Board settings and click Card layout.
- Add or delete fields as desired.
- Enable the Days in column toggle to display how many days an issue has been in the board columns.
but I don't have 'Board settings' as an option.