To generate images in newly defined image sizes, install and activate the Regenerate Thumbnails plugin. After activating the plugin, go to Tools » Regen. Thumbnails.
Pressing the regenerate thumbnail button will start generating new image sizes defined by your theme or in Settings » Media page. It may take a while, depending on how many images you have on your website.
You can also regenerate image sizes for selected images in Media » Library.
Regenerating images sizes does not affect your original upload. It creates additional copies of images in the newly defined sizes.
It also does not delete previously created image sizes. You can delete the sizes if you are sure that you haven’t used those sizes anywhere on your website.