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

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