wordpress Post image thumbnail 404 after change thumbnail size in setting – media

Solution for wordpress Post image thumbnail 404 after change thumbnail size in setting – media
is Given Below:

I have some problems with media thumbnails in WordPress.
Default thumb old size is: 400×300

I created many posts with this thumb size. But for some reason, I changed thumbnail size in setting – media from 400×300 to 400×210.

after changing size, all posts not change thumb from 400×300 to 400×210 and can’t show images.

https://i.stack.imgur.com/IHl6k.jpg

How to solve this or any plugins can fix it.

Thanks for All.