Can my ex change my daughters last name?

I was not on my daughters birth certificate, and her last name was that of my ex. I took my ex to court for a paternity test, and before the test came back, my ex changed the babys last name to her new boyfriends last name. The test came back positive that the baby is mine and I have been fighting for partial custody. Is there any way to change my daughters name to mine now that paternity is determined? Or even getting it changed back to her mothers last name would be better than her new boyfriends name! Was it legal for her to change it after I took her to court?