You can buy a [product|IF301-017|replacement trackpad] for your laptop and [guide|97072|follow this guide] to replace it.
Look through iFixit’s [[Electronics Water Damage|Electronics Water Damage guide]] and make sure there isn’t any corrosion or other damage internally. If you notice minor corrosion or liquid damage to the trackpad you could try fixing it with the techniques outlined on that page as well, before buying a replacement.