If you are asking about downgrading the packages of LXD to 2.13 and you do not mind clearing up any existing container in order to start again from scratch, then it should be possible.
You would need to uninstall/purge the current LXD packages, then install the specific version of LXD you need.
However, having a look at https://launchpad.net/~ubuntu-lxc/+archive/ubuntu/lxd-stable shows that the older LXD versions are not available anymore (currently, only LXD 2.15 is there). Therefore, you would need to find the set of LXD 2.13 packages from your /var/cache/apt/archives/ (if they are still there).
You could look into the "backports" repository if it has the 2.13 version, https://packages.ubuntu.com/xenial-backports/lxd
However, it does not have it, it has 2.14.
There are some other, more involved, ways to try to get LXD 2.13. But why get this specific version of LXD?