There's nothing to update. The mRNA allegedly encodes for the 'spike' protein, which is the infection vector. The theory at the time was that by targeting that protein specifically, if the virus were to mutate, it would loses it's ability to infect because it could no longer successfully bind to the ACE2 receptors.

In practice, the efficacy is a fraction of what the trials stated it was. It didn't lose efficacy due to the delta variant, the manufacturers just lied about the efficacy by cherry picking the data.

The only option they have now is to blame people's immune systems for lack of response, thus the 'boosters' are needed.