code: @RequirePOST
      public void doDowngrade(StaplerResponse rsp) throws IOException, ServletException {
          if(!isDowngradable()) {
              sendError("Jenkins downgrade is not possible, probably backup does not exist");
              return;
          }
  
          HudsonDowngradeJob job = new HudsonDowngradeJob(getCoreSource(), Jenkins.getAuthentication());
          LOGGER.info("Scheduling the core downgrade");
          addJob(job);
          rsp.sendRedirect2(".");
      }
docs: Performs hudson downgrade.