How do I open up my microsoft document as a open office document?

okay, my microsoft ran out of its trial and i didn’t want to go and buy it so I just downloaded open office which is pretty much the same only it was free, but now the problem is that now when i try and open up a document off my usb pen it still opens up as the old mircosoft that has ran out and i can’t use it, so how do I make it open up as a open office document?

please help 🙂