For posts there even is an easy way to change this in the backend:
Settings → Permalinks
The problem is that for pages this isn’t a built in feature. Here is another question that might give you a hint:
Add .html (dot HTML) extension to custom post types
The reasons why I proposed using redirects instead of changing the permalink structure are:
- You stay within defaults in WordPress which avoids unwanted side-effects and always makes it easier later on.
- Non-html URLs are more common nowadays (and more beautiful if you ask me, but this of course is a matter of taste)
In case you decide to go the redirect route you can add this to your .htaccess
file:
RewriteRule (.+)\.html$ /$1/ [L,R]