and go through a set procedure of making 3 contacts in sequence...
here is a Reto link showing how. You could also take the watch in and have a Seiko service center do it or anyone familiar with the procedure.
If the seconds hand ticks over in 2 second increments, the battery will need changing soon and you are given 3 minutes to install the correct battery. If you take longer, then the software for setting the calendar must be reset via the above procedure! good luck haha!