Last week arrived the 15.2.1 version of iOS, delivered in particular with two bug fixes concerning the Messages application and the use of CarPlay. The previous edition, meanwhile, already dates from December. But if you’re up to date, then you can’t install it again: Apple has blocked rollback, as has been the case with iOS 15.0.1 since October.
It is always possible to perform the operation in a roundabout way, but it is of course not recommended. Especially since with iOS 15.2, a patch comes to protect the phone against a flaw of security major related to HomeKit.
iOS 15.3 soon in final version?
Remember also that iOS is also available in its version numbered 15.3 recently, but only in beta at the moment and therefore potentially with some annoying technical issues.
When iOS 15.3 is released for everyone, the fate of iOS 15.2.1 should then be similar to that of iOS 15.2: after a few days (about a week), it will no longer be possible to download this edition.