Sorry if this is a stupid question but I'm new to KDE Plasma (Steam Deck owner) and don't know how to simply uninstall a program I installed over Discover.

Thanks in advance!

Try this:

Open Discover
First, try searching for the name in the search box.

If it is not showing, you might have to try different search terms. It may help to click on the "installed' button, then search again, to limit things, or maybe just browse the "installed" items.

Can you tell us what you installed?

This worked for me, thanks!