Depends. As usual, it depends on the injury. If we're talking about just contusion without fracture, then just time, perhaps warm compresses to help resolve bruising, and pain relievers. If there are fractures, typically it's just time and stabilization. Fractures involving the mandible and maxilla, or dental fractures require a lot more care and rehabilitation.
Wait for healing. For the most part, there is no specific rehab after facial injuries. Mostly, it is just waiting for the skin and any underlying injuries to heal, which can take 6 to 12 weeks. If there is nerve or muscle injury, your doctor may prescribe some facial exercises, but this is very uncommon.