I'm trying to figure out a formula that would automatically change the values
in the 3 different cells when 1 month has been reached.

for example:

A1=$2100.00
B1= $25.00
C1= 36 (months remaining)
C1= Date

so $25.00 is a monthly payment so once the date reaches one month I want
$2100.00 to deduct 25.00 and the months remaining to deduct 1 month. and i
want this to be something that is automatic so each time a month is reached
the amount in A1 will go down until it reaches 0.