Swelling like that there is usually pus. Does she have mucous coming from her nostril, it's hard to tell in the photo.
How long has it been since you brought in new birds?
A peck/injury to the eye, getting some type of debris in it, poor ventilation and respiratory disease are some common causes of eye swelling.
General care would be to flush the eye with saline, remove any pus that you see. Sometimes a warm compress is needed to start loosening/working the pus out (chicken pus is semi-solid (cheese like) and needs to be removed. Flush the eye well and apply an ophthalmic ointment like Terramycin (apply 2X a day). You can also order eye drops like Ciprofloxacin online.
If she has any hardened mucous in the nostrils, try to get that out too. It's always a good idea to look inside the beak for any mucous, plaques/lesions/canker also look at the
Chonal Slit for any obstruction.
If it doesn't start to improve in a couple of days or if she worsens
after you have been treating the eye as mentioned above , then she may need antibiotics. Tylan50 can be found at most feed stores-dosage is .20ml per pound of weight given 2-3times a day for 5 days in a row. It can be given orally or by injection.