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If you go to Settings > General > Software Update, does an update show there? If so, you can tap on it to start the process.
 
The latest os will show up as per post #2. If you’re thinking of ios 15 then you can’t yet. But:
Devs can access the beta now,
Public can access the beta next month, or
Full release in the autumn.

*edit* if you don’t understand the repercussions of it all then wait until the proper official release in the autumn.
 
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