Assuming you already ahve the Floquet ports and the PBC (master and slave) correctly set.
1- In the Project manager tree, SELECT slave 1
2- Then Change the properties of theta and phi accordingly. ) For instance to change theta = 0 deg for normal incidence or theta = 30 deg. Please see an illustration in the picture (FP HFSS).
3- Another option to change the incidence angle is to double click on slave 1 to open the Phase delay tab, and then change the properties of theta and phi (see picture FP Phase Delay).
Just to add to Mr. Lamine's answer, you may assign a parameter ( say $theta) instead of 30 in the second figure, then go to parametric and set a range of all incident angles you want for your design.