My father is the owner of the house, and on the second floor area there is a bathtub over the top of the water damaged ceiling on the first floor that i currently live, my father told me that these pipe may leaking thus thus cause a giant bubble on the ceiling, however i don't want to brother the tenant my self and thus my father only check it once and of course i didn't saw it with my naked eyes. So, base on your experience/knowledge, what will be the different way for me to replace the water pipe on second floor without the need to damage the bathtub in some of the most common situation?
First of all, the water damage in your unit could simply be because the upstairs people aren't properly closing the shower curtain when they shower, not the result of leaking pipes.
If there is a leaking pipe, you will not be going through the tub to fix / replace it - that would destroy the tub. You'll be opening up the wall on the other side of the tub, or the floor under the tub, or more likely both.
As for tracing the pipes - if they're copper you might be able to use a metal detector but its generally most reliable to open the walls up and trace them. You might be able to tell where the pipes are running by studying the basement where they enter the walls but that's still not guaranteed to help.
If the ceiling on the first floor is bubbling then there must be a leak somewhere on the second floor or somebody is just splashing a LOT of water in the bathroom. Because you claim it is directly under the bathtub then you will need to find the source of the leak.
Many older homes have an access hatch in the wall that can be removed to get to the plumbing behind the tub fixtures. This is the first and most obvious place to look for a leak.
If this checks out then inspect the second floor bathroom for any mold or water damage on the walls or floors. This may give you clues to what may be happening. Perhaps there isn't a leak at all and they are being careless in the shower. If you do find mold or damage then you may have located the source of the leak.
At this point once you have confirmed a leak you may want to remove the damaged parts of the ceiling or walls to get a better inspection of the plumbing that you can't see. Don't worry too much about this because you will need to repair the damaged ceiling or wall plaster eventually after the leak has been fixed.
If you still cannot find the source of the leak then you should call a plumber.