How can I change the C# language permanently in V2017.1?

I get compiler errors with C# language version 4, so I want to change to “Default”. I can do this im Mono Develop, but this is lost after restart. Mono saves the project, but this doesn’t help. Changes are lost after restart of Unity. (I tried to change the relevant files by notepade++, but no success.)

Try going to Edit → Project settings → Player under “Other”, under the header “Configuration” you will find an option called Scripting Runtime Version. Switch that back to "Stable (.NET 3.5 Equivalent). This should set it back to that version.

Thank you for your answer. The project settings “Configuration” was set on “stable”. Ich changed to “experimental” and reverse, but it didn’t help.