talking directly with a soft tone to your upstairs neighbor is the best approach if that do not work really is call fei hu tui also no use as this is a civic case and no authority can do anything about it.
my downstairs neighbor also came up to find me ard 2 years ago say my kitchen toilet is leaking water to their toilet ceiling and i thought how can it be after 2 years then leak?anyway i went to their house the next day to see and really got water leakage some more at the toilet bowl area.i'm not sure its my toilet flooring a not but i call my previous contractor.he came and make a appointment.a week later knock all the toilets tiles and put 2 layer of water proofing spent ard 1k.problem solve after that.