An exe file is always binary, meaning it´s already compiled. You need to disassemble - reverse engineer - it, and that will most likely just bring it back to C++ or into assembly code, which won´t help you if only know VB 6.
Also, you may be prevented, legally, from editing that exe file, and in addition, if it is part of an application, you may need to edit several files not just the EXE.
Essentially, what you´re asking can be done, but it seems like a pointless task and if you only know VB 6 you will be in for some long hours doing what you want.