Regenerate thumbnails generates new sizes of the previously uploaded images by creating images in the newly defined sizes. 

1. Visit /wp-admin/ > Plugins > Add New

2. Search "Regenerate Thumbnails" Plugin and click "Install Now" button

3. Plugin will start Installing

4. Click "Activate"

5. Visit /wp-admin/ > Tools > Regenerate Thumbnails

6. Please make sure you selected checkbox this way:

- DESELECT:  Skip regenerating existing correctly sized thumbnails (faster).

- SELECT: Delete thumbnail files for old unregistered sizes in order to free up server space. This may result in broken images in your posts and pages.

6. Click the "Regenerate Thumbnails For All XX Attachments" button

7. Regeneration will start, please wait

8. Done