Adjusting a Grand Seiko steel bracelet1 is fairly straightforward once you know how to do it. All you need is a 1.2mm screwdriver. I used the one that came with my Sinn 556i, which happens to be the same size.
Instead of a typical bracelet where the pin is held within the bracelet by friction, in Grand Seiko steel bracelets it is held within by two minuscule screws, one on each side of the bracelet. Simply have to remove either one of the screws, and shake the watch (somewhat vigorously — don’t accidentally knock it against something) to release the pin. You don’t have to remove both screws, although it might be easier that way2 especially if you find the screws easy to remove. Be extremely careful not to strip the screws, or it might be a costly mistake.