Daniel-
An editor /cannot/ unilaterally change policy.
An editor (everyone is an editor on Wikipedia, so you might as well say "a user") can change policy pages if there are no objections to them doing so. They should use common sense to determine whether their change would require prior discussions, or be reasonably minor or non-controversial not to. The same principle applies to edits of non-policy pages. Be bold in updating policy pages.
Regards,
Erik