I have a pre-build soundfont but I don't like some instruments and I want to replace soundfonts samples with mine but I don't find a way to replace them: there is one and if yes how?
You shouldn't really think of replacing the samples but creating an new instrument and replace the old instrument in the preset with the the new instrument.
This is the recommended way because the instrument settings - like loop positions, envelopes, low pass filter - are normally connected to the samples and anyway need to be checked and adjusted. Additionally the okd samples may be in monophonic format and the new samples in stereophonic format.
BUT - there are two alternatives: 1) if you rename the sample files to have exactly the same name as in the SoundFont, and add these files, Vienna will ask if you really want to replace the sample. 2) a better alternative is to use the split editor. Add the new samples. Select a split for which you want to change the sample. Go to the split editor below the table of splits. Choose "Wave data" and press the button Reassign down right. Select the new sample from the list.