it is not possible to get the original c++ code from compiled executable because c++ gets compiled to native machine code. imho the best way to reverse native executables is by disassembling. Check OllyDbg (http://ollydbg.de/) or IDA / IDA Free (https://www.hex-rays.com/products/ida/). There are c++ decompilers but i've never used them.

If the stuff you want to update is hold as a resource try Resource Hacker (http://www.angusj.com/resourcehacker/).