You can do this in the WordPress Admin panel. Navigate to Settings > Permalinks and choose “Default.” This will reset your link structure to default. After saving, you should clear your cache and it should work. However, this will not change your html pages or any other links that is hard coded into the template files.
Your post pages and any other pages in WordPress is PHP driven but for some reason you this is not the case for you, you could try using the “Custom Structure” option and paste this code in: /%post_id%.html