Teaching using bad material
As a compromise, given you are staying with the existing slides, consider saying things like "For purposes of this course, the definition of X is [what the slides say] but many people use [more conventional definition]". That way, the students know what they should assume during the exam, but have been warned that they will see different definitions in the future.

In thinking about contributing to updating the slides, you would be doing it to help future students, not to help the instructor who wrote them.