Source: https://gabmus.github.io/HydraPaper/#uninstalling-hydrapaper-flatpak
You can uninstall HydraPaper from GNOME Software or Discover.
Alternatively you can run one of these commands:
If you installed HydraPaper using GNOME Software or system wide: flatpak uninstall org.gabmus.hydrapaper If you installed HydraPaper using Discover or using the --user option: flatpak --user uninstall org.gabmus.hydrapaper
NOTE: Uninstall Flatpak