How to calculate the payment on a loan with an annuity interest after partial early repayment with a decrease in payment?

Question

Fintech

The formula for calculating the annuity interest seems to be simple and is described on many resources related to loans, for example, in the article Formula and calculation of an annuity payment on a loan . In code it looks like this:

function annuity(credit, procent, mounths) {
    const p = procent / 100 / 12;
    return credit * (p + p / (Math.pow(1 + p, mounths) - 1));
}

There are two options for partial early repayments:

1. Reducing the term
   
2. Payment reduction
   

The first is obvious, the amount of debt decreases and it is quite simple to calculate it.
But how is the payment reduction calculated?