However, if you have a late-run M4-P or any M6 model, then you will need to use a special RF adjustment tool and in the case of the M6, now remove the Leica red dot logo above the lens mount to uncover the recessed adjustment "screw".
Yes this is a nuisance and was deliberately introduced by Leica to stop weekend warriors from doing their own adjustments. You cannot use any old screw driver or torx tool to do the adjustment either, only the special tool Leica make and supply - "Adj. RF M4P, M6" #1500-0000-1279 - for approx $US 250 (ouch).