When you do change tyour user admin, do not use "admin" for your user name. This is the most common mistake . Hackers will break your login quickly, all they need is your pasword. To prevent that, change t=your admin user login name to more secure name, this way its harder to guess
Create a new account using wordpress dashboard.Assign that user administrator access.Log out from default admin account. Then log in to your new account. Go to users menu. Delete old administrator or admin and assign all posts to current user. Thats' it. You have new admin and there is no existence of 'admin' user.Pretty safe way to do that.