I have got one Wordpress blog site and I use permalink as:
%category%/%postname%.html
I want a change as:
%postname%.html
And now old urls (https://www.example.com/category/post-name.html) 301 redirect to (https://www.example.com/post-name.html).
How can I redirect with use .htaccess file?
Just go to wp-admin, then /settings, then /permalinks and select "Post name". More info on that here: https://codex.wordpress.org/Using_Permalinks
For 301 redirects you can write your own to your htaccess file but this is probably not as easy as using a plugin like Simple 301 Redirects.