code: @RequirePOST
      public HttpResponse doSiteConfigure(@QueryParameter String site) throws IOException {
          Jenkins hudson = Jenkins.getInstance();
          hudson.checkPermission(CONFIGURE_UPDATECENTER);
          UpdateCenter uc = hudson.getUpdateCenter();
          PersistedList<UpdateSite> sites = uc.getSites();
          for (UpdateSite s : sites) {
              if (s.getId().equals(UpdateCenter.ID_DEFAULT))
                  sites.remove(s);
          }
          sites.add(new UpdateSite(UpdateCenter.ID_DEFAULT, site));
  
          return new HttpRedirect("advanced");
      }
docs: Bare-minimum configuration mechanism to change the update center.