Not enough information to help. What make+model of computer do you have? If it came with 32-bit Win7, are you sure it isn't a 32-bit processor? Normally, 64-bit systems come with 64-bit Win7 unless they were ordered with the 32-bit OS specifically.
Not all 64bit processors come with 64bit OS's. My i3 came with Windows 7 Home Premium 32bit and I'm now running Windows 8.1 Pro 64bit on it, 3 years later.
Maur.stape, there's no way to upgrade your infrastructure. Your computer either supports a 64bit system or not. If not, you have to upgrade your CPU to a 64bit-capable one. If it is, what you need to do is install a 64bit version of Windows 7.
Get a Windows boot disc/drive, boot from it (by selecting the disk/drive from the boot order menu in BIOS during pre-boot) and choose a 64bit version of Windows during the setup process. Once installed, you'll have 64bit Windows and you'll be able to use all of your RAM.