I'm trying to demonstrate that the Green function is even:
$$\Delta(x)=\frac{1}{(2\pi)^4i}\int \,d^4k\ \frac{e^{ikx}}{k^2+m^2}$$
$$\Delta(-x)=\frac{1}{(2\pi)^4i}\int \,d^4k\ \frac{e^{-ikx}}{k^2+m^2}=\frac{1}{(2\pi)^4i}\int \,d^4k\ \frac{e^{i(-k)x}}{(-k)^2+m^2}$$
Now I would like to add a minus sign in the measure, like $d^4(-k)$, so to change variables like $k'=-k$ and having the demonstration complete. But I don't know how to insert that minus sign in the measure, do I have to add a minus sign also in front of the integral to do so?