First, you have to remove the old version. Then you can upload the new one. See https://explore.researchgate.net/display/support/How+to+make+content+private+or+remove+it for instructions how to remove. See also "How do I add a full-text to my publication page?" in https://explore.researchgate.net/display/support/How+to+add+research, but you probably know this already.