You can check on their website under support / firmware / your camera whenever you feel like an update, don't believe there is a subscription service there.
Alternatively, you can subscribe to the relevant firmware thread here, in which case you will get a notification whenever people start discussing new firmwares, and Viofo often posts pre-release firmwares there for testing and feedback, before they are put on the Viofo website.
If you actually have a serious problem that requires a new firmware, then you could ask Viofo Support to notify you when a fix is available, but they don't normally release firmware with serious problems, so I don't think they will often do that.
I don't think the App checks for updates, and many people don't like apps that are tied to, or spend their time sending data to the internet, or get stuck for a while unsuccessfully trying to contact Viofo servers when you don't have any mobile signal, before they will let you use them for what you want to do, or insist that you enter all your contact details and phone number so that they can verify who you are and keep track of you before you are allowed to use the app, so I don't expect that to change! Other dashcam companies do have apps like that.