Disable C# 6.0 Support in ReSharper

While using ReSharper, it suggested "Enable C# 6.0 support for this project". I foolishly clicked on it, and now as advertised it's giving me suggestions for C# 6.0 - which then give me errors as I am not using C# 6.0 in this project.

How can I disable C# 6.0 support, returning it to how it was before? (Preferably without having to individually ignore specific suggestions)

Click the project node in the Solution Explorer. Then look in the Property Grid (F4). You'll see a property named "C# Language Level". Set that to "Default" or your desired language level.

TO disable it at once across the solution rather than per project level, please add below in .sln.DotSettings

<s:String x:Key="/Default/CodeInspection/CSharpLanguageProject/LanguageLevel/@EntryValue">CSharp50</s:String>

it would disable resharper 6.0 features/give you error

EDIT

as per comments, in case you dont have the sln.DotSettings file at all, then you need to create one with below contents

<wpf:ResourceDictionary xml:space="preserve" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" xmlns:s="clr-namespace:System;assembly=mscorlib" xmlns:ss="urn:shemas-jetbrains-com:settings-storage-xaml" xmlns:wpf="http://schemas.microsoft.com/winfx/2006/xaml/presentation"> 
    <s:String x:Key="/Default/CodeInspection/CSharpLanguageProject/LanguageLevel/@EntryValue">‌​CSharp50</s:String>
</wpf:ResourceDictionary>