Runtime requires the version in the installation folder. So you cannot change that assembly, but you can still use your version of the assembly by adding it to the application config file located in the installation folder for Pega Robotics: OpenSpan.Runtime.exe.config. You would have to add the assembly reference to the <runtime> section. Here is a article explaining it in full detail:

https://support.microsoft.com/en-us/help/837908/how-to-load-an-assembly-at-runtime-that-is-located-in-a-folder-that-is

<configuration>
   <runtime>
      <assemblyBinding xmlns="urn:schemas-microsoft-com:asm.v1">
         <dependentAssembly>
            <assemblyIdentity name="MyAssembly2"  culture="neutral" publicKeyToken="307041694a995978"/>
            <codeBase version="1.0.1524.23149" href="FILE://C:/Myassemblies/MyAssembly2.dll"/>
         </dependentAssembly>
      </assemblyBinding>
   </runtime>
</configuration>