The circuit allows allows the charger to keep 6-V batteries in shape during the winter season, when the vehicle or machine is not used. The nominal output voltage of the L200 regulator is calculated from
 

Uout = Uref * ( 1+ ( R4 + P1 ) / R3 )

where Uref is 2.77 V (typ.).

 So, change R3 to 2.7 kOhm, and R4 to 1.8 kOhm,  these values allow the charger's output voltage to be adjusted between 6 V and about 7.7 V. The LED series resistor, R5, is lowered to 1 kOhm. Because of the lower output voltage (6 V instead of 12 V), the dissipation in the L200 may be on the high side if the original transformer is used. If you need the present 6-V version only, the transformer is, therefore, best changed into a type with a 9-V secondary. Suitable types are the Block VR13/1/9 (9V/1.44 A), the Monacor (Monarch) VTR12109 (12 VA), or the Block PT13/1/9. The latter is a short-circuit protected type (more expensive than the others, unfortunately). All three transformers are rated at 230V primary voltage, and fit on the printed circuit board. Also do not forget to change the fuse to a 63-mA type. As with the 12-V version of the charger, the current limiter is actuated at a level of 0.5 A.

   Schematic