how to convert an app?

Impossible?! May you should start it from scratch using a specific framework but tell us more about it. You said it exists on a website, so it was built using JS?

How do I convert an app (in this case a game) from the website

There is no magic tool (except just accessing the web site from an app, but such a scheme will not be accepted on the appstore).

so that I can easily publish it on the app store

ignoring easily, here is what you could do (unfortunately, there is no easy way to the AppStore, it requires 🥲some perspiration)

You have to design the app, using the website as your specification.

• You have the different pages on website -> create View controllers (VC) for each of them
• design each VC from each webpage content
• create the transitions between VC.

Note the Appstore requirement 4.2 in the guidelines:

4.2 Minimum Functionality

Your app should include features, content, and UI that elevate it beyond a repackaged website. If your app is not particularly useful, unique, or “app-like,” it doesn’t belong on the App Store. If your App doesn’t provide some sort of lasting entertainment value or adequate utility, it may not be accepted. Apps that are simply a song or movie should be submitted to the iTunes Store. Apps that are simply a book or game guide should be submitted to the Apple Books Store.

Good luck.