In this instructable, I will show you how to adjust the rear spring preload on a Honda CBR 250R.
The default setting is Position 2 which is on the soft-side. A softer setting increases riding comfort so that your spine absorbs less of the energy transfer from bumps, but takes less additional load for the suspension to bottom out. On the other hand, a stiffer setting will be capable of taking on a heavier load before the suspension bottoms out but will provide a less comfortable ride.
The only tool you will need is a pin spanner that already comes with the motorcycle. I hope this helps.