Apple released iOS 15.2.1 last week with a fix for an issue with some third party apps on CarPlay not responding as well as an issue that could cause some Messages sent that contain an iCloud link not to be sent. Apple also fixed a pretty important security issue with HomeKit in this release too.
However now that iOS 15.2.1 is out, Apple has now stopped signing iOS 15.2, making any possible downgrades impossible. This is to help encourage users to keep iOS up to date and ahead of any security issues.
If not so already, your device can be updated by going to Settings > General > Software Update.