For Xbox users, there is a special addon program which is updated regularly whenever new WASM planes are released. It should install/update automatically whenever you launch MSFS on Xbox, but if you were in suspend or quick launch mode, then it may not check for an update. Make sure you fully quit MSFS and re-launch it to force the update. If that still doesn’t work, try restarting your Xbox console.
You can also try the steps described in this thread: