This depends on the cause of the mass, where it is and what type of tissue. It really cannot be dealt with except by a veterinarian.
It could be a fluid filled cyst from various causes, a tumour, scarring, an overactive organ caused by some other issue, or many other things. Only a vet can assess this properly, and they will be able to advise what is the best approach for treatment.
Do not delay in seeking treatment. It is unlikely to get better by itself.