If understood correctly, you’ll have to follow this Codex instructions: Using a pre-existing subdirectory install.
If you already have WordPress installed in its own folder (i.e. http://example.com/wordpress) then the steps are as follows:
If that’s your case, you’ll learn how to make WP work as if it were on the root, even being installed on a folder. If your actual .htaccess only have the redirection you mention, you can delete it, otherwise make a backup so you can merge it with the new one (from WordPress).