The following methods can be used to let the pus out of a sore after it has become pus-filled: 1. Surgical methods can be used to let the pus out of a sore that has become pus-filled. There are many good ointments available in the market nowadays, which can be applied on top of the boil to shrink and mature the pus, and also to break the epidermis on top of the boil so that the pus can flow out. 3. In some cases, the sores can mature and the pus can break down and come out.