How To Remove Hard Lump Under Armpits?

My 6 year old daughter noticed a lump under her left armpits couple months ago. I figured it would go away. Today when she got home from school she said it was hurting her. I looked at it and saw it had a white head so I applied a little pressure and a little drop of pus came out. And also what looked to be dead skin was kinda attached to it so I softly pulled it And it came right off but she still has the hard lump. What do I do?