The offset of a wheel is the distance from your hub mount point (the point which the wheel attaches to the car) to the exact centerline of your wheel (when viewing the cross section of the wheel).

Offset cannot be adjusted, as this is intrinsic to the wheel itself. You either need to get different wheels with the correct offset for your car, or you will need to roll your fenders to accomodate your (I assume wider) wheels.

Beware that if the offset of the wheel is not correct for your car, the handling can be adversely affected.

Good luck!