How do I treat a hard lump in my armpit?

The first thing to do is to find out the nature of the lump and then treat it according to the causative factor.
If the lymph nodes are enlarged due to inflammation, they can be treated with broad-spectrum antibiotics or antibiotics for the corresponding pathogen.
If it is a benign tumour such as a lipoma, or a malignant metastasis, then surgery is usually required.