If an update is released for a product you own, it will be shown in the updates tab of your Spitfire Audio App
Click on the update button, and you will be prompted to select the location for this install, which should be your existing library folder for the relevant library.
If you use the default install location or set a custom one, you might see that the relevant file path for your folder is already shown on this screen.
You can always manually browse to the folder by clicking on the folder icon.
Once selected, you'll see the selected file path confirmed on this screen before proceeding.
Click on update and your download will start.
Once the download is complete, your library has been updated successfully.