Assume that you have a monthly installment for ten years, your loan is 10000, and the interest rate is 1% but it changes at the beginning of February rising 1%, and calculate the new installments.

10000/10=1000 yearly installment

1000/12 monthly installments

But the interest rate changed from February, so the first month's installments should be 1000/12, and the second month's installments should be 10000-(10000-1000/12)*3%

I want to know how to calculate that in a formula. I think we use AIFV=(1+0.03/12)^20 something like that to calculate