I think you're using Visual Studio. First of all, check in the properties of all that FileHelpers referenced if the Specific Version is set to False (if not, do it). Try to clean and build back.
If the problem persist, in the app.config of your solution, in the configuration
section, add
<runtime>
<assemblyBinding xmlns="urn:schemas-microsoft-com:asm.v1">
<dependentAssembly>
<assemblyIdentity name="FileHelper" publicKeyToken="3e0c08d59cc3d657" culture="neutral"/>
<bindingRedirect oldVersion="0.0.0.0-2.0.0.0" newVersion="2.0.0.0"/>
</dependentAssembly>
</assemblyBinding>
</runtime>
In this way you should be able to force the used version to the new one.