Ios 15 software size is 2.51gb and my iphone has 3.6 gb free storage but the storage issue isn’t solved ?? What i do ??

3.6 GB is not enough. The 2.51GB is the installer; the phone needs twice as much as that to unpack the download and install the update. So you need to free up an additional 2 GB, or update using your computer.

Go to Settings/General/Software Update, take and post a screen shot of that page.

To update over the air you must be connected to a Wi-Fi network (and not a personal hotspot) unless it is an iPhone 12 series with an active 5G network connection. You can still update using a computer with a wired or Wi-Fi Internet connection.