What is Reddit's opinion of Vastar Watch Adjustable Opener Back Case Press Closer Remover Repair Watchmaker Tool and Watch Case Back Opener Repair Remover Holder Tool?

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After some quick googling it looks like that's a quartz watch so it has a battery and requires far less maintenance than a mechanical watch. I would say 98% of the time it's incredibly easy to swap out the battery on your own. Since it's a dive watch, though, you may need to find a replacement seal for the cover if you want to keep the extreme water resistance.

Remove the case back (you may need to get a special tool but it's very cheap). Once the back is open you can find out what type of battery is in it. Search Amazon and you'll most likely find what you'll need and I'm guessing it will be relatively cheap.

Here's some videos of people changing Tag batteries. They aren't your exact watch but the principles are the same:

https://www.youtube.com/watch?v=Ff6grbcFLBE

https://www.youtube.com/watch?v=9PXMBRd1KaY