How can i improve pain caused by fallen arches/flat feet- which exercises to do?

Wear orthotics: To take strain off of foot, role foot over a frozen water bottle to reduce any inflammation. Consider surgical correction of the foot to,resolve your issue.

Need arch support: There are some strengthening exercises you can do for your arches but they mostly don't improve your pain long term. You really need arch support in your shoes to help hold the arch up. I recommend you see a podiatrist for evaluation.

Progression possible: Traction on the medial plantar branch of the tibial nerve can lead to tarsal tunnel syndrome. This can be confirmed with EMG-NCS testing. In addition to pain you might experience tingling in the instep radiating up to above the ankle. Arch supports as others have advised is the way to go. Those Dr Shoals machines you stand on can help fit you with the right arch support. If that fails see foot DR.