Hello there.
To lower a fever, rub a swab of rubbing alcohol on her paw pads, and stomach, as this will cool her. If the fever is higher than 103.5, wrap her in a cool damp towel to lower the temperature quickly.
If your kitten has a fever it is likely due to an infection and I would highly recommend a veterinary visit, as kittens are frail and can deteriorate quickly.
I hope this is helpful.