WordPress sites are a little more complicated than sites made with just HTML files, because the files that it runs from are PHP files. However, you may need to upload a full HTML file to your WordPress site -- for instance, to migrate pages from a different HTML site without loading them into the normal post database, or to verify to a third party that you are the owner of the site. In such cases, you can upload an HTML file to the WordPress Media Library.
Log in to your WordPress admin area. Click "Media Library" from the menu on the left, then click "Add New."
Click "Select Files" on the screen that appears. A window comes up, allowing you to browse your hard drive for the HTML file that you want to upload. Locate the file, click it once, then click "Open" in the bottom right corner. Wait for the file to upload, which for a normal HTML file should not take long. Note that WordPress will return an error if the HTML file you try to upload is blank. If all you need to do is upload an HTML file with a particular file name -- for example, to verify that you are the owner of the site -- type the following into the file before uploading:
Copy and paste the URL of the file, located next to the "File URL" field. If you need to link to the page from your main site, this is the URL you should use. You can also fill out the "Title" and "Description" fields to make your uploaded file easier to find.
Click "Save all changes" when finished.
If the upload screen in the Media Library does not display correctly, it may be because you do not have the latest version of Flash installed. If this is the case, download the latest version for the browser you are using from Adobe's website. If you cannot install Flash for some reason, use the "Browser Uploader," located at example.com/wp-admin/media-new.php?flash=0 -- replace "example.com" with the URL of your blog.