I can’t find anywhere in settings to do this!
You can 
You’ll need to disable and re-enable the PIN unlock function, and it’ll prompt for a new PIN.
I just added this to the docs here: add note for changing PIN by tgreer-bw · Pull Request #603 · bitwarden/help · GitHub - thanks for asking!
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Many thanks! I should have thought of trying that myself!
Cheers!
It also makes sense, now, why I am asked on my iPad app to enter a new pin number each time I open the app!