You can help your cat that has a cold by keeping the room they're in steamy or use a vaporizer in the room to keep moisture in the air.
Apple Cider Vinegar can also be given to the cat to help with their cold, but other than that the most you can do is wait it out and let time pass.
The cat should be over it's cold within a week or 10 days. If the cat still has a cold after 10 days you might need to take the cat to the vet to get some antibiotics.