Regenerate Thumbnails allows you to regenerate the thumbnails for your image attachments. This is very handy if you’ve changed any of your thumbnail dimensions (via Settings -> Media) after previously uploading images or have changed to a theme with different featured post image dimensions.
You can either regenerate the thumbnails for all image uploads, individual image uploads, or specific multiple image uploads.
You can see here for more details.
Tools > Regen.Thumbnails
Regenerate All Thumbnails