What can to do for my daughter's swollen lip?

Lip. Swollen lips can be due to many causes, e.g. Insect bite, injury, infection, systemic illness etc. But, most important and may be serious if it is from allergy. Allergic reaction gives rise to swollen lips, tongue and inside oral cavity which can lead to respiratory difficulty and arrest. See your pediatrician if you are not sure or not comfortable.
Depends on the cause. It depends what caused your swollen lip. It can be due to allergy to meds, insect bites or foods. It can also be due to infections like cold sore or another infection and may need antibiotics. You should check with your doctor as if it is due to allergy you need to find out what caused it and may need to carry Epipen if it keeps happening as it can be life threatening. Meanwhile yu can take benadryl (diphenhydramine).