We had the IGRF out of date error earlier this year and the *fix* is a software update. 8.20 is that update for the certified world.
Easy peasy; I highly recommend you bring the maintenance manual with you as there is a chance some of the settings may get erased and you will need to reset them. If your installer did not provide you with the configuration sheet from the manual, I also recommend you take the time to copy that down before you do the update.
Logbook entry required, we recorded ours as database update for G5 AHI (S/N) and G5 HSI (S/N) completed IAW Garmin G5 Maintenance Manual & ICA (P/N), ops check complete.
Takes all of 20 mins for 2x G5s, including paperwork and ops check.