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

Dear aplusmerchant,
based on your photos, it looks like you would benefit from dermal fillers. If you are looking for a more permanent solution, then fat transfer might be an option. If you are considering a treatment, I would suggest you to consult a board-certified plastic surgeon. Only after a thorough examination you will get more information and recommendations.
Daniel Barrett, MD
Certified, American Board of Plastic Surgery
Member, American Society of Plastic Surgery
Member, American Society of Aesthetic Plastic Surgery

Forehead dent treatment. I prefer Voluma for this as it is reversible and lasts 2 years. Fat can leave bumps and can be irregular and NOT easily repaired  

The dent to which you refer is the result of infracture of the brow bone in that forehead area. So rather than having a convex shape like the other side it is concave. Because it is a result of fracture the overlying soft tissues will be stick by scar into the healed fracture segments. While injectable filler and fat are one option, a more effective approach with be endoscopic release of the soft tissue and filling in the bone defect with either bone cement, a hand carved implant or ideally a small custom implant. A 3D CT scan of the forehead will reveal the full extent of the bony depression. 

The forehead can be indented when enough force strikes to break the outer table into the frontal sinus.
This can be repaired using bone cement or methylmethacrylate, bone grafts and other methods. Personally have used meth on the deck relate and found it to be excellent. It’s like putting bondo  on a car dent. 
It might be worthwhile to be evaluated if you have a history of a skull fracture. He didn’t say what the nature of the accident was.
Fillers will be more precise and more forgiving. Long lasting fillers like Voluma or Radiesse should work very well for you.
For a true bony contour irregularities that includes a fracture into your sinus a bone reconstruction may be more appropriate.
In Sacramento consider talking to the plastic surgeons at UC Davis medical Center.
Plastic surgeons who work and level one trauma centers or academic centers are more likely to have experience with the management a facial fractures.
You may or may not have had a fracture. It’s worthwhile to get properly evaluated.
Fat transfer is an option but it is a bit imprecise and unpredictable. personally I would not choose that option.
By the way the treatment of facial fractures or deformities from facial trauma are generally covered by medical insurance. This may however not include ongoing use of fillers.
Best, 
Mats Hagstrom MD