What photo editing software do you have access to?
It's a difficult job to make a significant difference that will help an enlargement. Localised sharpening of eyes, teeth and the texture of clothing can help - that's the stuff with details.
If you have software that supports layers and/or a 'filter' call high pass a better result can be achieved but there are no miracles. Additionally two weak runs of the high pass filter generally is more effective than a single blast at high strength.
Rob