Usually dedicated server OS can be installed in two ways: first way is to select a OS while submitting the order in which case the server will be delivered with OS preinstalled. And the second option, which is described here, is to install or reinstall operating system for your server from administration panel.
As a first step you will need to access server info and actions "My Services" >> "Dedicated servers" >> Select your server from the list the server that you want to install OS for >> Select tab "Manage server".
In the new view you will have to select OS that you want to install from drop down list and credentials:
WARNING: Please be sure you backed up your old data BEFORE this operation, otherwise your data will be permanently lost.