You need a tool to open the caseback. They're readily available for a couple of quid on amazon/eBay but you need to be very careful as it's easy to scratch the caseback if you're not sure how to do it carefully.
If you're thinking of selling, pop to a local watchmaker and have them remove the caseback, take a picture and oh it in your listing.
If you're keeping it (I would!), I'd wait until you send it to service. Both the watchmakers I listed above will take pics at various stages of service so you can see what the movements like.
I assume you're UK? What region/county? I'm in Yorkshire but have no qualms sending my watches to Christian (WatchGuy) or Mitka who are also in the UK but further south.
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