how to manually upgrade from 15.04 to 15.10 so I can upgrade fully

Getting the error an upgrade from vivid to xenial is not supported.
I need to know how to manually upgrade from v 15.04 to 15.10 so I can then upgrade to 16.whatevs as per usual.

I have tried

sudo do-release-upgrade 

and used the software updater program, and both return the same error. I do not want to skip versions from 15.04 to 16.x, I just want to manually install 15.10 from my current version, and that should allow do-release-upgrade to fix the rest.