After every installation go to installed directory (ex. C:\5.6.31\xampp, C:\7.1.18\xampp) and start the "setup_xampp.bat" and you should see something like
this.
You can make shortcuts of "xampp-control.exe" on your desktop (right click on "xampp-control.exe" Send to -> Desktop) and rename shortcuts for ex. "xampp 5.6.31" and "xampp 7.1.8".
Start XAMPP control panel with double-click on "xampp-control.exe" or previously created shortcut and start Apache and MySQL servers.
To test installiation open your browser and type 127.0.0.1 or localhost in the location bar. You should see XAMPP start screen.
Do not open more then one XAMPP control panel.
XAMPP uninstall? Simply remove the "xampp" Directory.
But before please shutdown the apache and mysql.
That's all. You can use different php versions opening corresponding XAMPP control panel.