@kimooi , Inn, Fitbit says the battery is not changeable, Below I posted a trouble shooting link and a link for the tear down of the watch so you can see how to access the battery, Purhaps the battery will have a Part# which you can google to get a replacement. Good luck.
I hope this helped you out, if so let me know by pressing the helpful button.
[guide|42344]
http://help.fitbit.com/?p=surge