Step 1: Go to ‘Plugins‘ then click on ‘Add New‘
Step 2: Search for ‘AJAX Thumbnail Rebuild‘ then click on ‘Install Now‘
Step 3: Activate the plugin by clicking on ‘Activate Plugin‘
Step 4: Wait for it to download
Step5: Go to Menu ‘Setting‘ > AJAX Thumbnail Rebuild
Enforce regenerating thumbnails
1. To finally enforce regenerating the thumbnails pick Tools > Regen. Thumbnails from the navigation menu on the left.
2. Click Regenerate All Thumbnails to regenerate new image sizes as defined by your theme or on the Settings > Media page.
3. Watch the conversion progress. Note that you may need to wait for quite a bit of time if your media library is large.
4. Check out the post. As you see, despite the plugin’s name it regenerates all the image sizes, not just the thumbnails.
Regenerating Thumbnails for selected images
When you need to regenerate sizes only the certain images, you can do that in Media > Library section. To do this you would need to click Regenerate Thumbnails link near the image you want to alter – and make sure you’ve switched to the list view first.