Most Shimano dual pivots have a small set screw on top of the brake caliper on the right side. Dura Ace and Ultegra have an allen screw, 105 and below has a Phillips head/slotted screw. It's a screw that will accomplish the recentering you need. If my memory's good Shimano puts loctite on the screw so it may not turn too easily. This will provide fine (small) recentering adjustments.
If the brake is way off, loosen the recessed bolt on the rear of the fork (the one that holds the brake on the fork) a few turns, apply the front brake hard, and retighten the screw. Then use the approach above to fine-tune the centering.
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