You can sometimes buy mini screw driver sets from a pound shop for a urrr, pound.
http://www.watchbattery.co.uk/Replace-Watch-Battery.shtml covers most of it, and a thin smear of petroleum jelly on any O rings / seals should preserve any 'waterproof' capability.
@MrE started
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http://www.watchbattery.co.uk/Replace-Watch-Battery.shtml covers most of it, and a thin smear of petroleum jelly on any O rings / seals should preserve any 'waterproof' capability.