You are aware that the iPhone SE and the green-color versions of the iPhone 13 and iPhone 13 Pro will come out soon. These devices will have iOS 15.4 pre-installed in them as they hit the stores. If your current iPhone is compatible to run iOS 15.4, go to the Settings app, and click on General. There, click on Select Software Updates, and follow the on-screen instructions for downloading and installing iOS 15.4.