demote
To move someone to a lower job or position.
To demote someone means to move them to a lower position or rank, taking away some of their authority or status. When a military officer gets demoted, they might go from captain back down to lieutenant. When a baseball player gets demoted from the major leagues to the minor leagues, they're sent to play with a less skilled team.
People usually get demoted because they aren't performing well in their current role, they made serious mistakes, or the organization needs to restructure. A store manager who consistently misses sales targets might be demoted to assistant manager. A student who struggles in an advanced math class might be moved to a regular section.
A demotion (the noun form) often feels disappointing or embarrassing because it signals you weren't succeeding at the higher level. It's the opposite of a promotion, when someone moves up to a bigger role with more responsibility.
Sometimes people use the word more casually, like when your favorite song gets demoted from your top playlist, or when leftover pizza gets demoted from dinner to tomorrow's lunch. In sports, whole teams can be demoted (or relegated) to lower divisions if they finish at the bottom of the standings.