To uninstall Atomic Red Team you can simply delete the default installation directory which is <BASEPATH>\AtomicRedTeam where <BASEPATH> is C: on Windows or ~ on Linux/macOS.

And if you'd like to be very particular, you could also uninstall the powershell-yaml module that gets installed with the Invoke-AtomicRedTeam installation. First, close all PowerShell sessions, then run the following command from a command prompt (Windows) or terminal (macOS/Linux).

powershell -NoProfile -Command "Uninstall-Module powershell-yaml" # this is for Windows
pwsh -NoProfile -Command "Uninstall-Module powershell-yaml" # this is for macOS/Linux