How to rewrite /index.php to /app.php

How to rewrite /index.php to /app.php

I have this in my .htaccess:
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteCond %{HTTP_HOST} ^www\.(.*)$ [NC]
RewriteRule ^(.*)$ http://%1/$1 [R=301,L]
RewriteEngine On
RewriteCond %{REQUEST_FILENAME} !-f
RewriteRule ^(.*)$ app.php [QSA,L]
</IfModule>
I also want to rewrite mypage.com/index.php. How can I do it?