This is not possible. See https://explore.researchgate.net/display/support/Reviewing%2C+featuring%2C+and+editing+your+research : "Certain types of research cannot be changed to another type, including data and preprints. To change the type, you will need to remove the research item and reupload it as the correct type." I recommend to leave the preprint entry as it is and add the published version as a new entry. See also https://www.researchgate.net/post/I_want_to_change_some_publication_types_How_to_do_it .