Easy ! Remove cages, undo and remove plastic dust cap ( use circlip pliers with care ) Remove old grease from pedal body and inside of cap.
Fill pedal body and inside of cap completely with grease and screw cap back into the body. You will have pumped the fresh grease into and around the two rows of ball bearings forcing the old grease out towards the crank end of the pedal axle. You can repeat this operation if required
On a good day the pedals should feel like new, however if there is any slight play in the pedal, you will need to tighten the outer bearing track slightly.
Shimano make a special tool for this, but it can be done as explained above, with patience. On a really bad day the pedals will still feel rough, which may indicate that the ball bearings ( and track ) have become worn. The ball bearings are a special Shimano size ( 2.2mm dia. ) and not available. You can fit the nearest available larger size ( you will need 4 rows of 11 balls in total ) pm me if you get this far