Simply go to the following URL and enter your login details (NIC + password)
You will then see the following menu with your VPS on the left-hand menu:
On your VPS page, simply select the "Reinstall my VPS" button to open the choice of OS window:
After clicking, you will see the list of OS available upon reinstallation of your VPS:
You must then confirm your choice:
Once this stage has been confirmed you can't go back, so we advise you to back up your files before confirming this reinstallation. Once the reinstallation is complete, you will receive an email containing the new access details for your VPS.