How do I treat bumps on my baby’s face and neck?

Depends. There are different kinds of bumps and rash. There isn't one treatment for all of them-- it depends on what the bumps are. Usually, though, bumps on a young baby's face don't require any treatment at all. Get an exam with your pediatrician to see what the bumps are and what you ought to do next.
Unnecessary to treat. In most infants, these go away without treatment; in very severe cases an oral antibiotic can be used.