Hi

In doing phd, what will happen when after we do the experiments the results shows not promising? Maybe the hipotesis that we want to proof cannot be proof. Is there any condition like that? or it must proof the hipotesis

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Proving the null hypothesis is still a valid piece of work but you would need to be able to address criticism of the choice of the original question (I'm having some of that trouble myself - it's very common) and probably publighing opportunities would be compromised. If it doesn't work at all - again you need to justify your choice and thoroughly explain why.

It's also wise to try to leave some time for a 'subplot' i.e. a related expt that is more likely to yeild interesting results. I put in a subplot in my 2nd year when it became clear I would have serious trouble getting enough data to draw a firm conclusions about the main issue. It means my end PhD will be a collection of related papers rather than one central piece but that's common with experimental PhDs.

ok..
I heard some people change their hipotesis during the writeup so that it is tailored to their current work. Is it ok like that?

As long as the hypothesis is valid and the work is relevant then it should be OK.