In order to change the operating system on your VPS or reformat it, you will need to login to the OpenVZ control panel. The login for that control panel should have been emailed to you after ordering the service.
Once logged in, simply click the "Reinstall" icon and select the distribution you want. Click "Reinstall" at the bottom of the page and confirm to allow the reinstall.
Note: This will delete all data from your VPS!
Wait for a few minutes for it to complete and you can login to your VPS once again. The root password should be the original password that was emailed to you (or the one that is shown in the billing portal). You can also change your root password from the control panel as well should it not work.