src/index.js

-import { createApp, h } from "vue";
+import Vue from "vue";
 import { createApolloProvider } from "@vue/apollo-option";
 
 import Demo from "./components/Demo.vue";
@@ -8,10 +8,13 @@ const apolloProvider = createApolloProvider({
   defaultClient: createDefaultClient(),
 });
 
-const app = createApp({
-  render() {
+new Vue({
+  el: "#app",
+  apolloProvider,
+  render(h) {
     return h(Demo);
   },
+  beforeCreate() {
+    this.$.appContext.app.use(this.$options.apolloProvider);
+  },
 });
-app.use(apolloProvider);
-app.mount("#app");