The process differs from a desktop Windows Edition where you are prompted to download the Net Framework Package from Microsoft. Such process wouldn't work on your Windows Edition.
To proceed, you would need to proceed with the following steps:
1) Login to your Remote Desktop Server
2) Open or Search the Server Manager
3) Once Opened Click the Top Bar on 'Manage' and Select 'Add Role and Features'
4) On the next Stage click on Next
5) Then Leave the default Selected Option 'Role-Base or Feature Based Installation' and click on Next
6) At the next step, you may leave the selected options default 'Select a server from the server pool' and your VPS will be highlighted by default being the localhost.
7) At the Server Role Stage, you may also click on Next.
8) At the Features Stage Select The Net Framework 3.5 or / and Net Framework 4.6 and click on Next
9) At the next stage, you may select 'Restart the Destination server automatically if required' which will prompt the Windows server to restart upon completion in which case, you would need to save your running application data and click on Install
That's it you have now installed Net Framework on your Windows Server.
In order to benefit from any others version (4.7, .4.8), you would need to run the Windows updates which will prompt you to install the newest versions.