code: public BoxRequestsFile.PromoteFileVersion getPromoteVersionRequest(String id, String versionId) {
          BoxRequestsFile.PromoteFileVersion request = new BoxRequestsFile.PromoteFileVersion(id, versionId, getPromoteFileVersionUrl(id), mSession);
          return request;
      }
docs: Gets a request that promotes a version to the top of the version stack for a file.
  This will create a copy of the old version to put on top of the version stack. The file will have the exact same contents, the same SHA1/etag, and the same name as the original.
  Other properties such as comments do not get updated to their former values.
  
  @param id    id of the file to promote the version of
  @param versionId id of the file version to promote to the top
  @return  request to promote a version of a file