How? Do you treat? Postural orthostatic tachycardia syndrome?

With difficulty. This is often difficult to treat successfully. First, you need a tilt table test to confirm the diagnosis. If confirmed, volume expansion by eating a high salt diet and drinking plenty of fluids may help. Beta blockers, Fludrocortisone (florinef) and midodrine (proamatine) are used with variable success. Good luck.
Multiple treatments. Postural orthostatic tachycardia syndrome (pots) is a disorder of the autonomic ( think automatic ) nervous system where the heart rate rises dramatically with standing. There are many associated symptoms with a wide spectrum of severity. For mild cases, simple dietary adjustments are helpful, but most require medical treatment. You should see a physician who specializes in the treatment of pots.