You need to change the size in settings to the new one. Then you have to regenerate the images to that new size because when you change the size within WordPress they are still saved as the previous size. For that you can use a plugin like : Regenerate Thumbnails
After you have run the generate you can simply delete the plugin after.
Description of the plugin:
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.