500 internal server error after upgrade

[nomoreturn]

Thanks Lynne i was waiting for your reply yes i talk to my host and they said there is nothing wrong or changed on my account since i host my site ( its been more than 3 years now my site on net) and i think they are right because before upgrade it was working perfect and after upgrade got this error here is what they said about this error

The most common cause of a “500 Internal Server” error is the permission
level of a website file. If you open up File Manager and look in the folder
where your website is installed, you will notice that each file is assigned a
three-digit number. These numbers represent the permission level of the file.
The higher the number, the more permissions granted. However, very high
permission levels are security risks. HostPapa servers will not use files if the
permission levels are set too high.

If you see files or folders with permission levels set to 777, they need to be
changed to 755. Likewise, if you see files or folders with permission levels set
to 666, they need to be changed to 644.

And i dont know which folder or files need to be permision changed so please help me in this thanks