code: public GitlabBuildVariable updateBuildVariable(Integer projectId,
                                                     String key,
                                                     String newValue) throws IOException {
          String tailUrl = GitlabProject.URL + "/" +
                  projectId +
                  GitlabBuildVariable.URL + "/" +
                  key;
          GitlabHTTPRequestor requestor = retrieve().method(PUT);
          if (newValue != null) {
              requestor = requestor.with("value", newValue);
          }
          return requestor.to(tailUrl, GitlabBuildVariable.class);
      }
docs: Updates an existing variable.
  
  @param projectId The ID of the project containing the variable.
  @param key       The key of the variable to update.
  @param newValue  The updated value.
  @return The updated, deserialized variable.
  @throws IOException on gitlab api call error