A similar question has been already asked about this subject:
Is it possible to "upgrade" from a 32bit to a 64bit installation?
Like it has been explained, the upgrade of Ubuntu (32-bit) to Ubuntu (64-bit) is for professional-level sysadmins therefore, I recommend you to be really careful with that. (Debian based systems 32-bit to 64-bit).
So, if you have a separated /home partition, the easiest solution would be to overwrite your / partition by doing an installation of Ubuntu 16.04 (64-bit) over it.
With that being said, if you absolutely want to do the migration and you don't have a /home partition, there is a straightforward way to do this migration: Quora - Migration 32-bit to 64-bit
P.S. If you don't mind loosing all your data, you should just then install Ubuntu 16.04 (64 bit), it'll be the quickest and easiest way possible.