How can I play a game that I have already install in my desktop?
As long as you obtained the software legally it’s fine. Do not pirate on this platform- if you are found out you will face serious consequences.
If it’s from a distribution platform like Steam, GoG, Humble Bundle or other you can just download their respective applications, log in and download the game. If it’s an older game that requires an installation disk you can follow this video guide here:
That is all I can think of in terms of legal acquisition of games- as long as it’s from an official source and it is legally obtained it shouldn’t be an issue.