If the bearing is adjustable the left-hand cup will have a lock-ring around the outside. Turn this anti-clockwise to unlock the cup. This is easiest with a lock-ring spanner that fits the notches on the ring exactly, you can also use a universal tool with one tooth or a tap it round with a hammer and an old screwdriver.
Once the lock-ring has moved back the cup can move. If it has hexagonal flats turn it with a spanner. If it has holes use a peg spanner.
If the bearing is too loose screw the cup inwards clockwise. If it's too tight screw the cup outwards anti-clockwise. Make small adjustments and check between them.
Once the bearing is in the target zone hold the cup still with a tool and screw the lock-ring down against the frame to jam the cup in place.
Make a final check and replace the crank.