You will need to convert your C++ unmanaged code to 64bit.
Be aware that this may be non-trivial depending on the complexity of your C++ codebase.
See these links for more information:
Microsoft - 64bit systems
Intel - Collection of common 64 bit errors
As for C# code, you can easily set the platform target to x64 and build the modules. You need to ensure that any external unmanaged modules are x64 as well.
Microsoft - Convert 32 bit managed to 64 bit
You would also need to convert your web app to 64 bit. Not that you could leave it as "Any CPU" if you were not using any external unmanaged libraries.