It's how they normally come off and unless you can see a hidden grub screw, I can't see how else!
You can look up the replacement part on a site like 4ourhouse.co.uk and find the exploded parts drawing which should show you the part and how it is fitted!
Well, I could see a lot of staining on the photo, at the back of the dial which is why I said wd40... Looked like rust on the piccie
Or... Having just studied pic again, it might be that the fascia panel comes off and over the dial, leaving it in position! A service manual would show you, if you can hunt one down online!
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