If you installed the package using `sudo apt install signal-desktop` as instructed at signal.org's download page, then there should be another (system-wide) desktop file at `/usr/share/applications/signal-desktop.desktop`.
If you right-click the `Signal (env)` menu item, does it offer any "properties" option or similar? That might tell you more about where it lives.
It looks like Cinnamon has a dedicated menu editor, accessible by right-clicking the Mint icon in the bottom-left corner, selecting `Configure...`, then selecting the Menu tab and clicking on `Open the menu editor`.