Hey @alapierre
To the best of my knowledge you can’t edit contact owners in the property settings but would manage users via the “users & teams” settings. Here you can either deactivate a user or remove them from the account.
Usually I would check how many contacts and companies are assigned to a user. You can either built a workflow to reassign exisiting records or do bulk reassignments, it depends on the number of records and if the reassignment rules are simple or a bit more complex.
One important thing to keep in mind. Have you disconnected the Salesforce integration? If not, this might overwrite whatever changes you make in the owner assignments.
Hope that helps.
Frank