If you installed with the setup program, open Apps & features, choose Xackup v2 and uninstall it. The rest of this page is for the PowerShell install.
For the PowerShell install, download PowerShell installer package from the Downloads page, unzip it, and run it in PowerShell as Administrator.
.\Uninstall-XackupDesktop.ps1
This removes the XackupAPI service, the program files, the shortcuts and the Apps & features entry. Your data is kept, so a later install picks up where you left off.
Delete the data too
.\Uninstall-XackupDesktop.ps1 -Purge
-Purge also deletes C:\ProgramData\Fungusware\Xackup: jobs, saved servers and locations, and settings. This cannot be undone. The upgrade backups in C:\ProgramData\Fungusware\Xackup-backups are not deleted; remove that folder yourself if you no longer want them.