Linux

Just install the package of DE and switch to it through the login page (display manager).

Fedora do provide a "group" of packages to install COSMIC Desktop. To install it:

sudo dnf install @cosmic-desktop

And now you should be able to select it on login.

Fedora KDE uses SDDM as login/display manager. I've never used it so I don't know where to click to switch DE, but maybe it should be the "Other" button.

Fedora Workstation uses GDM (Gnome Display Manager). To switch DE click the cogwheel in the bottom-right corner.

Edit:

By reading the official Fedora documentation I've found out that there are "special groups" named "environments".

To list all of them:

dnf environment list --available | grep -i desktop

So to install COSMIC:

sudo dnf install @cosmic-desktop-environment

I don't know what's the difference, but this is what the official documentation says.

Edit 2:

By doing:

dnf environment info cosmic-desktop-environment

It's clear that "environment"s are groups of groups of packages. So it could include stuff you don't need, like libreoffice group.

Word of advice, it's best to make a new user for a new DE, lets you go back easily and avoids conflicts, basically necessary with KDE -> Gnome.

If you're actually already on an atomic fedora (as evidenced by rpm-ostree) you can have a look over here. There are also a few other images around but most seem old and unmaintained.

I don't have Fedora around, but in general just install all the desktop environments you wish from package manager. Most (if not all) login managers support changing the environment on every login. I don't remember exactly how that works on plasma login manager, but there should be a pretty obvious menu item to pick from before inputting your password.

You obviously need to have autologin disabled in order to see the selection screen.

If you read the document there's this GUI tool for the same :

https://docs.fedoraproject.org/en-US/quick-docs/switching-desktop-environments/#_using_switchdesk

Are you aware COSMIC is still in beta? It's very buggy and not ready for regular use.