If you "google VL2020 with solder tags" you get a number of options to buy from about £3 upwards.
Then you need a soldering iron with a fine tip. Just un-solder the old one, trim and bend the tags on the new one to fit, and then solder it in (check the polarity). You may find that some de-soldering braid or a de-soldering pump helps although I would be inclined to just lift the tag clear of the board using tweezers as soon as the solder melts.
It shouldn't be a difficult job if you are reasonably practised with a soldering iron. If you're new to electrical soldering then you might want to have a play around with something that doesn't matter first, then try to put as little heat into the job as you can get away with when you do the battery change.