Seiko was founded as a small watch shop in Tokyo by Kintaro Hattori in 1881. The modern Seiko company is one of the world's largest watch makers. Most Seiko watches operate using a quartz movement, powered by a small battery. You will need to remove the case back, using either a case knife or a case opener, to access the battery.
Place the watch face down on a soft cloth to prevent scratching.
Fit the case opener into the notches on the case back. Open the case back, referencing the instructions that came with your case opener (it will need to be opened in a counterclockwise direction).
Remove the metal dustcover underneath the case back (if any). Simply use tweezers to pull up one side of the small metal cover.
Lift the battery contacts that are holding the battery down, gently using the tweezers. Remove the battery carefully and set it aside.
Install the new battery, positive side up, then push the battery contacts down to the original positions.
Check to see if the watch has restarted. If it has not, use the tweezers to restart the watch by connecting the positive side of the battery with the small hole marked AC inside the watch. This will start the watch.
Hold the watch in your hand inside the soft cloth to protect the watch while working on it.
Locate a good place to slide the case knife under the watch back. Many Seiko watches have a small dent that makes it easier to place the knife.
Slip the knife under the back slightly then apply pressure and move the knife sideways. This should lever the back off the watch.
Follow the directions for the screw-off back, starting at step 3.
Be careful when using metal tools around a watch, as it is easy to scratch the case.
If you are nervous about opening your watch, take it to a local jeweller to have the battery replaced.