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.
1. Navigate to Plugins > Add New. Search for Generate Plugin and Install.
2. After active Regenerate Thumbnails plugin, go to Tools > Regen. Thumbnails and click on Regenerate Thumbnails button.