Head over to the relevant service section in the user area (by clicking on "Linux SSD" on top of the page).
Select the server you wish to reinstall
In the server page open the "Rebuild" tab on the left side
Select the OS and press the "Re-Install VPS" button
Let the VPS installation complete
All of your data stored on the server will be lost
In case of Windows VPS, you will need to finish the setup procedure via the VNC console, like you did when the server was initially deployed