Only the current owner can change the ownership of a report to another user. They can do this by clicking the Share option in the Analytics header bar and configuring the ownership options in the Share Report pane.
Administrators can also choose to change the ownership of reports while deleting the current owner. At the time of deleting the user, they have an option to change the ownership of the reports from the user being deleted to another user.
If the current owner of the report is already removed from the system, then there's no option to change the report ownership. In such cases, reach out to the support team for help.