How do I remove black mold on the window sills?

Wipe with white vinegar to kill the spores. It's probably formed due to condensation.

Bleach

Home Depot has mold remover.

Plain old white vinegar. Vinegar kills mold spores, bleach does not.

https://www.servicemaster-restoration-cleaning.com/2017/03/mold-myths/

Mold loves moisture, that’s for sure! And it doesn’t matter if it is cold refrigerator moisture or hot summer moisture from windows inside a house, mold will find a way to live and grow. In your situation, I would strip the window of curtains, blinds, sheers, draperies and ornaments. Clean the window, blinds and woodwork with warm water with vinegar and a little Dawn. Dry everything with dry cloths. Launder or dry clean the textiles (fabrics), and become a sleuth. Check for air vents that blow on the window and change the direction, usually by moving a lever, or by turning the vent cover upside down and res crewing. Be sure a refrigerator or other source of moving air or moisture is not blowing on the window or sill. Check that the window itself is secure and closes completely. Check the ceiling tiles so that none leak. With all the fresh linen on the window, make sure vents and dehumidifiers are not blocked and can do their jobs. Dust that window sill often with a dry cloth, and check the blinds and trim for mold each time. An ounce of prevention is worth a pound of cure. Happy Spring!

Hi Tina - There are several ways to get rid of mold, but most importantly, you need to address the root cause to keep it from returning. This article focuses on the bathroom, but is an overall great article that covers methods of how to remove mold, if bleach is safe to use, and how to prevent it from coming back. Hope this helps! Hugs, Holly

https://pinkfortitude.com/mold-in-the-shower/

Purchase Concrobium Mold remover spray from your home or hardware store, following application directions.

You need to run a dehumidifier set no higher than 40%, leaving it run until it reaches that point. All doors and windows to that area should remain closed.

Operate kitchen and bath fans regularly. Operate ceiling fans where installed. Insure adequate space between clothing, linens, books, etc. Place Damp Rid around your house.

Check attic for leaks. Install window and door weatherstripping as needed.

Scrub walls and rinse with white vinegar water.