Treatment for flat feet and fallen arches depends on the severity and cause of the problem. If flat feet cause no pain or other difficulties, then treatment is probably not needed. In other cases, your doctor may suggest one or more of these treatments:

Rest and ice to relieve pain and reduce swelling
Stretching exercises
Pain relief medications such as nonsteroidal anti-inflammatories
Physiotherapy
Orthotic devices, shoe modifications, braces or casts
Injected medications to reduce inflammation such as corticosteroids

I should have said another thing is that if pain or foot damage is severe, your doctor may recommend surgery. Procedures may include the following:
Fusing foot or ankle bones together (arthrodesis)
Removing bones or bony growths - also called spurs (excision)
Cutting or changing the shape of the bone (osteotomy)
Cleaning the tendons' protective coverings (synovectomy)
Adding tendon from other parts of your body to tendons in your foot to help balance the "pull" of the tendons and form an arch (tendon transfer)
Grafting bone to your foot to make the arch rise

You can take self-care measures to prevent or manage pain from fallen arches or flat feet. Wear footwear or shoe inserts that are appropriate to your activity.

When pain occurs, try at-home treatment of rest, ice and over-the-counter nonsteroidal anti-inflammatories such as ibuprofen.
Ask your healthcare provider or a physiotherapist to show you stretches that can prepare you for feet-intensive activities.
Limit or treat risk factors that can make fallen arches or flat feet worse such as obesity.
Avoid activities that put excessive stress on your feet such as running on roads.
Avoid high-impact sports such as football, tennis, rugby, basketball, or hockey if you find these trigger symptoms.
Know when to get help. When pain is severe or interferes with your activities, it's time to seek medical advice for a thorough examination and treatment.