How do i make a file check sytem?

Hi, i'm working on a installer. It works fine, but if someone is installing a newer version over the old version, the config file of the program will be replaced by a new config file. I want a script that detects if the config file is there, and if its so, he wont write a new config file. But if there is not a config file he must write it.

Can someone help me plz 🙁

ps. sorry for my bad english

SetOutPath "$INSTDIR"

IfFileExists "$INSTDIR\config.cfg" NoExtract

File "config.cfg"

NoExtract:

-Stu

thankz 😁