Updating Ubuntu Version of Homestead Box

Hello,

I am trying to get up to Ubuntu 16.04 for my Homestead box as I need to test a feature of it. I set up my box when it was still using 14.04.

After running vagrant box update 1575db9 I get the following result:

==> default: Checking for updates to 'laravel/homestead'
    default: Latest installed version: 0.5.0
    default: Version constraints: >= 0.4.0
    default: Provider: virtualbox
==> default: Box 'laravel/homestead' (v0.5.0) is running the latest version.

So it seems to be up to date. But when I SSH into the box and run lsb_release -a it tells me that the box is 14.04:

No LSB modules are available.
Distributor ID: Ubuntu
Description:    Ubuntu 14.04.4 LTS
Release:    14.04
Codename:   trusty

I don't know if the box is up to date or not - it says it is running the latest version, but it's still on 14? Will I have to create a brand new box or something, or is it possible to upgrade the OS this way?

Many thanks

Also I've never been able to use the homestead command, I always have to use 'vagrant' - not sure if this might effect something.