Much depends on the underlying reason for the swelling. Is this something recent or has it been a continuous problem? If it is due to an allergic reaction - insect bite or contact with a plant - you can give plain benadryl at a rate of 1 mg per pound of your dog's body weight. Do NOT use the benadryl (diphenhydramine generic) that also contains a decongestant.
If the swelling is due to a blow to the face or anything other than an allergy, then you need a veterinarian to evaluate the problem.
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