The firmware update begins automatically after you attach your battery pack to your iPhone. It can take up to a week for the update to be completed. Or instead of a week, you can connect your battery pack to your Mac or iPad with a Lightning-to-USB cable and complete the update in 5 minutes.
The MagSafe Battery Pack is compatible with iPhone 12 and iPhone 13 series for charging on the go. It can be particularly helpful in prolonging the short battery life of the iPhone 12 Mini. Of course, it is still better and faster to charge your phone with the 20W power adapter whenever you can.
You can check your firmware version on your iPhone in Settings > General > About > MagSafe Battery Pack.