My 5month old baby is having a fever, 101.4. What should I do to bring down the temperature?

Nothing. Fever may be beneficial to your baby, and unless they are obviously very uncomfortable because of the fever would not do anything. If you feel uncomfortable about it, there is pediatric Acetaminophen liquid preparation with dosage recommendations for weight and age, but would first try sponging baby down with tepid washcloth. No true medical need to inhibit fever of this level.
Fever. A fever that high in a 5 month old is very concerning. You should medicate with Acetaminophen - 5mg per kg dose every 6 hours - in addition, tepid baths will help. If the temperature doesn't drop - you should seek immediate medical attention.