Take whatever model it is you want and unzip the folder inside. If there's just one folder inside, you know it's the model folder. Sometimes there's more if it comes with art or UI, so make sure you have the right one if that's the case.

Open Vtube Studio! There should be a little menu of icons on the left side of your screen; if not then just double click anywhere and it should appear. Click the first icon, it should look like a little person and be a redish color. A new menu will appear along the bottom of your screen with all available models Vtube Studio comes with, this is how you switch between models.

In that same menu, all the way to the left should be 2 options "Unload Model" and "Import Model". Click Import, and it should open up a folder in your files. Drag and drop the model you unzipped at the start into it and voila, you're done! You should see it in the models menu immediately, but if not, just close the model menu and reopen it.