How can I fix this dent in my forehead from an accident? (Photo)

There are several approaches to correct and/or improve that contour irregularity. Filler injection with temporary or permanent synthetic is easiest, but may be temporary and subject to further contour problems, e.g. if you wear a hat that compresses it, may dent. Hairline incision to smooth bone (if that is the cause) and place alloplast like solid silicone or porous polyethylene or even crushed cartilage in temporal fascia graft other possibilities. Consider getting in-person opinions by consulting with board certified plastic surgeons in your area.