In the library view in Endnote, you can first search for the records you want to perform this search and replace and have those records “showing”. – (remember this can’t be undone, so you may want to back up your library by compressing it to an enlx file, or copying the library and associated .DATA folder to a new location).
Under Edit (in the windows version) there is a Search /Replace option (ctrl+R) where you can put https: and replace with http: (uncheck word match) and select the appropriate field (URL).