Option 1

The Regenerate Thumbnails Plug-in.

Option 2

wp command line tool

Regenerate all images, without confirmation.

wp media regenerate --yes

Regenerate thumbnails only.

wp media regenerate --image_size=thumbnail

Error

Error: Error establishing a database connection.

When working on localhost, use 127.0.0.1 instead of localhost in wp.config

define( 'DB_HOST', '127.0.0.1' );