I study by myself with the QFT, in the page 197 of book of Lewis H. Ryder (2nd edition), The author wrote that he take the functional derivative of equation 6.69:
$$\frac {\delta\widehat{Z}[\phi]}{\delta\phi}$$
where $$\widehat {Z}[\phi]=\frac{{e}^{iS}}{\int{{e}^{iS}}{\cal D}\phi}\tag{6.69}$$
and
$$S=-\int{\left[\frac {1}{2} \phi(\Box+{m }^{ 2 })\phi -{\cal L }_{ int } \right] { d }^{ 4 }x }.\tag{6.71} $$
The result in Eq. 6.72 is:
$$\frac { \delta }{ \delta \phi } \left\{ \exp\left[ -i\int { \left[ \frac { 1 }{ 2 } \phi (\Box +{ m }^{ 2 })\phi-{\cal L}_{ int } \right] } { d }^{ 4 }x \right] \right\} { \left[ \int { \exp\left[ iS \right] } {\cal D}\phi \right] }^{ -1 }\\= \left( \Box +{ m }^{ 2 } \right) \phi \widehat { Z } [\phi ]-\frac { \partial { \cal L }_{ int } }{ \partial \phi }\widehat { Z } [\phi ].\tag{6.72} $$
I don't understand how the calculating procedure taking place. I have known how to calculate the functional derivative to a functional, but I do not know how to take it to a functional integral like $\widehat{Z}[\phi]$. I would be most thankful if anyone help me.
PS: Is there are some detailed textbook or literature about this technique?