How to promote an old app? now it has 300 million users

Hi, my dear friends, I am developer apps, called AppLock from DoMobile.
It’s the link:

Now it has 300 million users and many functions, How to get more new users ? It’s hard for me.
What suggestions do you have? How to update it better?

Let us discuss more details about games, apps.

Hope your reply. My email is [email protected]