I tried many times to up date ios 15 but it doesn't work ,please solve this issue. Thanks,

To update all of these requirements must be met:

• 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
• The phone must be charged to at least 50%, and preferably plugged in so it stays above 50% during the update
• The phone must not be in Low Power Mode
• The phone must not be connected through VPN
• The phone cannot be in Airplane mode

Go to Settings/General/Software Update and post a screen shot of it.

Also, go to Settings/General/iPhone Storage and post a screen shot of that.

What series watch do you have? FWIW, a lot of people have problems updating a series 3 watch due to insufficient memory. In some cases, unpairing the watch, completely erasing it, then pairing and restoring the backup will resolve the issue after an update. (Simply unpairing and pairing again may not work.)

Unpair and erase your Apple Watch - Apple Support

Also, see fireyjack's reply to:

Can't update her series 3 watch. - Apple Community

Question was an iPhone update, not a watch update 😏