deb file of an application. I want to convert the same file into 32 bit .deb file. So how to convert 64 bit .deb file into 32 bit .deb
That's not generally possible.
If .deb file contains any architecture specific files (ie libraries or executable binaries), then you'll need to obtain 32 bit versions of those files, or rebuild the package yourself from source.
If the contents of the .deb are already cross platform, then it should have already been marked as Architecture: all by the creator.
If the creator mistakenly marked a cross-platform package as platform specific, then you could manually edit the Architecture entry in the control file yourself, but this seems like a rather unlikely situation.