Opening a Seiko watch requires the wearer to pop off the back, unscrew the back with a special tool or unscrew individual screws. Which procedure works best depends on the type of back on the watch.
For the snap-off type of back, insert a flat tool such as a small screwdriver into the gap in the cover. Twist the tool to pop the back off.
With a screw-off type of back, use a special case tool that fits into the individual slots on the back of the watch. Insert the lugs of the tool into each slot, and twist counter-clockwise to remove the back.
If the watch has screws holding the cover on, remove the screws with a small screwdriver.