Well, if it's typical, it should have three controls on the PIR.
1. Daylight sensitivity, which sets how dark it needs to be before the light activates at all
2. Timing, which sets how long it will remain lighted after being activated
3. Sensitivity which sets how sensitive it is to movement.
No 3 is the one you need to adjust, by trial and error. Also chaning the orientation of the PIR is worth looking at. Mine gets set off by cats running along the top of the wall opposite, and until recently, nearby tree branches blowing in the wind (I chopped them off).