From final release post almost at the bottom of the post it say
Get the Ubuntu MATE snaps
When the upgrade is complete and you’re logged in, open a terminal and execute:
> snap install ubuntu-mate-welcome --classic
> snap install software-boutique --classic
> snap install pulsemixer
The snap packages above are installed when performing a clean install of Ubuntu MATE 18.04, but are not automatically installed when upgrading from an earlier release.