I want to downgrade the ios 15.5 version

You cannot downgrade to a lower version of iOS.

Maybe, you can describe the issues that you may be having with the current version so that the readers of your post can try and help.

Axel F.

1) Try and Force Restart your iPhone EXACTLY as shown below and see whether that resolves the issue:

• Press and quickly release Volume UP button
• Press and quickly release Volume DOWN button
• Press and Hold the SIDE button until an Apple logo appears and then release the Side button (Can take up to 20 seconds. (DO NOT release Side Button when invited to Slide Power OFF). 

(2) If the above does not work then - Contact your carrier and try another SIM card.

Axel F.