How to deal with high heart rate?

Depends . On the underlying cause. There are many factors that are involved in causing a high heart rate. You need to be evaluated by your doctorand at least have an ECG done for starters.
Cause? Depends entirely on the cause. High heart rate is your body's way of dealing with increased metabolic needs. Of course there are many diseases that also increase the heart rate. The list is very long and includes, fever, infection, anemia, thyroid excess, cardiomyopathy, arrhythmias and many more.