Did you download your Win7 upgrade or did you buy a boxed copy? If you have a boxed version, check in the box. My Win7 upgrade came with both the 32-bit and the 64-bit DVDs in the box. Try using the other optical disk in the upgrade package. If only one disc is in the package, contact the source where you got the upgrade. Only the package for installing a new OS (OEM or full version) comes with just the one disc included. For those, you need to specify the type of system on ordering.
If you downloaded the upgrade, you will need to get a different upgrade file and burn it to disc or make a USB installer for that version of Windows 7.
To confirm your current version of Windows, click Start and choose the Control Panel. Click on System and Security and then on System. This page will display the Windows version and append x64 if you have the 64-bit version.
Alternatively, download the Windows 7 Upgrade Advisor, http://windows.microsoft.com/en-us/windows/downloads/upgrade-advisor
, and it will confirm if your system is compatible with Windows 7 and which version you need.
I hope this helps.
(even the Windows 8 upgrade came with both the 32-bit and 64-bit DVDs in the box.)