So, you are using another writing program but decide to switch to LibreOffice. You want to use it as your default. That means the main program that opens when you double-click on an associated file. Usually during the original install you do get a chance to set this. But, in this case you didn't.
Here is how to switch over. Open LibreOffice. Click on Tools in the top menu. In the box that opens click on General on the left-hand side.
On the right-hand side scroll down to the Windows Default apps. Click on it if you want to wade through that jungle, otherwise, just check the box under that says Perform check for default file association on start-up. Click OK.
Now close LibreOffice and Reopen it. This time you'll get a little popup box asking if you want to set .odp, .odt, .ods in LibreOffice by default. Just click OK for Yes. The Perform check on startup option, your choice.
OpenOffice doesn't seem to have these options at all. If you are switching to OpenOffice, right-click on a document.
Click on Open with then Choose another app.
In the box that opens click on the app of your choice, More apps if it's not listed and be sure to check Always use this app to open .odt files.
You could reverse this and use this right-click method to set LibreOffice as well. This procedure should also work for most documents, images and media files.