Put on a pair of rubber gloves to protect your hands and skin. When working with bleach, it's also a good idea to wear old clothing to protect your skin and good clothing from the bleach.
Mix the bleach with water in a bucket. To prevent the bleach from being too strong, always use 1 tbsp. of bleach for every gallon of water.
Unplug the water cooler before beginning to clean it. Twist the water cooler bottle to remove it.
Place a medium-size bucket under the water cooler's faucets to allow any remaining water to drain.
Pour the bleach solution inside the water cooler's interior reservoir. Have the solution soak inside the cooler for five to seven minutes.
Scrub the inside of the cooler with a sponge do clean and sanitize the cooler. Always scrub the interior walls as well.
Drain the water and bleach through the faucets and into the bucket.
Fill the interior reservoir with water to rinse it out. Empty the water again through the faucets and into the bucket. Do this at least four times to make sure the bleach has been washed away.
Remove any removable parts of the cooler and wash them with the bleach solution, too. Run the parts under clean water to rinse.
Put the parts back on the water cooler and add the new water bottle.