What is the reason behind pressure changes as we move at higher altitude also the cause of temperature change at higher altitudes?

Thank you for the question,

Noticing that temperature and humidity affect the atmospheric pressure, the gravitational acceleration has a significant impact too, and since the gravitational acceleration decreases with altitude, the pressure decreases as well. Add to that the impact of air density  and its effect. Details here.

Concerning temperature, It is known in physics that temperature decreases with respect to pressure decrease. Also, at higher altitude where humidity is low, the serre effect responsible of reflecting thermal radiation is weak. The combination of previous reasons explain the decrease of temperature at higher altitudes.