(P1,T1) -> to (P2,T2) then what will be the change in enthalpy for ideal gas?
The enthalpy of an ideal gas does not depend on pressure.  It is a function only of temperature.  So $C_pdT$ is always equal to dH for an ideal gas of constant composition.  The subscript p on $C_p$ merely indicates that $C_p$ can be measured directly by determining the heat Q absorbed by the gas in a constant pressure experiment.