Answer
If
lsb_release -a
, tells you about Ubuntu 16.04 means you have elementary OS Loki installed. Don't know why doesn't say Loki.
elementary OS is a distribution on it's own based on Ubuntu, as Ubuntu is based on Debian.
A Ubuntu system can be upgraded to another Ubuntu version because is a rolling release distribution.
elementary OS is not a rolling release distribution, hence you can't upgrade between versions. You have to install from scratch each version. Now if elementary OS were a rolling release distribution, you only could be able to upgrade to another elementary OS version not to a Ubuntu version.
You can experiment with it and try, but you'll be breaking a lot of stuff along.
Is like if you install Ubuntu 18.04 and then upgrade to Debian Sid, I don't believe you'll be able to do it cleanly nor expecting to have a stable system in the process. Is doable yes, but why you installed elementary then?
If you want Ubuntu, then install Ubuntu.
My outputs
$ lsb_release -a
No LSB modules are available.
Distributor ID: elementary
Description: elementary OS 5.0 Juno
Release: 5.0
Codename: juno
$ lsb_release -au
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 18.04 LTS
Release: 18.04
Codename: bionic
$ cat /etc/lsb-release
DISTRIB_ID=elementary
DISTRIB_RELEASE=5.0
DISTRIB_CODENAME=juno
DISTRIB_DESCRIPTION="elementary OS 5.0 Juno"
$ cat /etc/upstream-release/lsb-release
DISTRIB_ID=Ubuntu
DISTRIB_RELEASE=18.04
DISTRIB_CODENAME=bionic
DISTRIB_DESCRIPTION="Ubuntu 18.04 LTS"